Originally Posted by
BarracksSi
There's friction somewhere, and it could be just a dirty, gunky chain.
Two things I can think of that would happen with a nasty chain -- it could actually make the RD "open up" (this could also happen with dirty jockey wheels) which almost erases tension on the chain and lets it fall off, or the chain could fail to unstick from the front chainring and get caught under the FD cage.
Also could be a worn out chainring - if the teeth are sharkfinned the chain will act similarly to being gunked up. When backpedalling, the chain will hang on below the tangent point on the top, and then let go suddenly.