Originally Posted by FBinNY

Just to be clear, are you referring to the chain moving to a smaller sprocket when you backpedal, or does it also do so during normal riding?

There's a big difference because the first is relatively normal, and the second is probably the result of a poorly trimmed derailleur.

If you look at your bike, you'll note that in one direction the guidance provided by either derailleur is BEFORE the chain engages, but AFTER when running backward.

So, when you back pedal the chain has nothing to prevent moving to the smaller sprocket if anything gets it started. It doesn't take much to start the shift because the sprockets are intentionally cut to make shifting easy and the system is dependent of the RD to feed the chain to the right sprocket (which it apparently does the moment you pedal forward again).

Odds are the system is less likely to shift when you backpedal when the chain angle is favorable, ie. outer chainring with outer sprockets, or inner with inner. So, my best advice is to simply stop backpedaling in those combinations where it's a problem.