it is a common issue and it occurs due to the chain lining from the rear cassette towards the front cranksets, if the chain is in the highest or lowest cogs of the cassettes and cranksets then when you pedal in the backward direction, the chain skips and come off from the cassettes and the crankset and skipping of chain cause stuck in the derailer and the jockey wheels, it is the problem which arises due to the derailleur but it is because of the cassettes wheel and the crankset and the chain lining, higher or lower cogs make the chain angle and lining bent and you have to make the chain line straight from rear towards the font by bringing the chain in the middle cog of the cassette when the chain lining is smooth then you can easily pedal backward without skipping of the chain.
It also happens if the chain is dirty and nasty and you are not servicing and lubing properly your chain on weekly basis, then the chain stuck and causes friction with the cogs and teeth of the cassette and the cranksets, after every ride the dirt and any water penetrates deeply within the chain and acts as a barrier against the smooth rolling track of the chain.
When the chains become old then the internal widths and space of the chain links are rubbed and eaten by the teeth of the cassettes and also the bushings between the chain links start squeaking and losing after riding long mileages, these kinds of problems would arise, the better way is that you can keep care of your bike chain and after 1 year or 18 months replace the chain and buy a new expensive chain with a warranty to get rid of these kinds of irritating problems and issues.
If the chain is not stretched fully and it is loose tensioned then it also gives rise to skipping of the chain when you start pedaling in the backward direction, check the tension of the chain and also check the angle and lining of the derailleur hanger, it is common these hangers would be bent and deform and distract from their actual position, which also dislocates the track of the chain which is reaching down toward the derailleur after passing through the cassette, straight the angle of the hanger so that it should be parallel with the wheel and derailleur and does not cause any resistance when the chain goes downward the derailleur.
Rubbed And Bent Teeth:
All the teeth of the cassette cogs and chainrings are smooth and are equally sized for the smooth running of the chain on them, if any of the teeth are rubbed or bent then it will change the direction of the chain all the chain links will follow the skipping direction, you can inspect all the teeth of the sprocket and chainrings, check all the cogs if any cog is bent or broken then replace it with a new one, after that this issue will never arise.
Stuck Chain Link:
Sometimes the chain links break and the inner and outward plates of the chain bend and deform due to which these links do not align correctly on the teeth cogs of the cassette and the cranksets, you have to observe the broken or bent chain link as soon as possible and replace them for the smooth working of the chain with the sprocket and the chain ring.
Stuck Jockey Wheels Of The Derailleur:
These are small wheels aligning on the lower head of the derailleur, the chain passes through these wheels and then towards the front chain ring, after every week you have to service and clean the dirt and dust of these wheels and lube them properly because these wheels stuck and start creating resistance and opposing force when you pedal your bike in the forward direction these wheels would start rotating with the force and it also dissipates a lot of your energy and when you pedal backward these wheels do not move freely in the backward direction and it allows the chain to skip from the front chain ring cranks.
If the derailleur is not properly angled with the cassette and the wheel then it will cause friction and resistance and stops the rolling of the chain at intervals, it only happens when the derailleur is old and ruined or if it is non-compatible with the cassette and the wheel and chain, then it stops working and it also needs servicing and cleaning on regular basis.
Gear Shifting Problem:
When you start shifting gears from lower to higher and higher to lower cogs of the cassette then sometimes the chain will be stuck in between the cogs of the cassette and the chainrings of the crankset so further, you have to shift the gears for complete alignment of the chain in the specific cog. and if the shifters of your gears are new and you are pedaling in the backward direction then there are more chances of skipping your chain, because new shifters are installed with new gear cables and new cables are stretched, on every gear shifting their cables would stretch and relax, it will cause a big impact on skipping of the chain, you have to make the shifter and gear cable compatible with the cogs of the cassette and bike chain.
Chain Skipping From Higher To Lower Cog:
When you are backpedaling and the chain is skipping from higher to lower cog of the cassette then it is normal, it is not a real chain skipping issue because when starting pedal forward again then the chain again jumps from lower to higher cog of the cassette, it almost occurs with all the categories of bikes which include road biking, mountain biking, and gravel biking. it is normal for the rear cassette but if the chain is skipping from the front chain ring then it is the chain issue and you have to check the chainring and chain to solve this issue.
Lining Of The Chain Ring/Cransket And The Rear Cassette:
Mostly the chain skips and slips from the cogs of the cassette and the crankset teeth just due to the false lining of the front chain ring and the cassette of the rear wheel, if the chain lining is not correct and accurate due to the front and rear cogs then the chain will easily skip when you start pedaling backward, you have to check the location of the crankset and then the rear cassette with the help of a scale, even a half millimeter of false ling between the front chain ring and rear cassette would cause skipping of the chain.
Bottom Bracket And Spacers:
If you have inserted spacers for installing the BB then it will also cause the same issue of skipping the chain because the chain line does not stay correct with the spacers and the wrong installation of the bottom bracket, you have to look for a mechanic or if you have skills then check the bottom bracket if its needs spacers then insert the spacers if not then unmount the spacers.
Non-Boost Chainring On A Boost Bike:
Old and traditional bikes are non-boost bikes so all of their parts are also non-boost ad new expensive modern bikes are called boost bikes, parts of boost and non-boost bikes are different and non-compatible from each other in size. so if you have a new boost bike and you want to install a non-boost chain ring then it will cause the3 same issue of skipping the chain when you will pedal in the backward direction.
Directional And Non-Directional Chains:
There are two types of chains directional and non-directional and all the Shimano chains are directional chains so those riders and bikers who have installed Shimano chains on their bikes are facing the issue of skipping the chain because a directional chain means you do not have to pedal in the opposite non-direction, and if you pedal then the chain would skip.
If the hub is old and you have not serviced it for a long time then it would not rotate smoothly due to which it starts creating a resistance force when you rotate the pedal or rotate the wheel in the backward direction, clean and service the hub thoroughly, after that use a recommended lube for bleeding of the freehub.