Originally Posted by
cny-bikeman
FB is right of course, but you really should not need an expert to tell you that your problem cannot be related to the rear derailleur. As the chain comes forward off the rear cog and then goes directly to the chainwheel the rear derailleur can have nothing to do with whether the chain moves from one chainwheel to the other.
Well, it is possible that the OP was backpedaling when the chain jumped from the 50T to the 34T. If the derailleur is sloppy or the derailleur hanger is bent (like if you lay down the bike on the drive side), the reverse chain feed can be off, leading to auto-downshift when backpedaling.