Originally Posted by
Koyote
Hmm. On my Di2 bike, hitting a button while NOT pedaling will indeed move the derailleur, just like on a mechanical system. So I'm not sure that this is the issue. But I sure would like to see the OP come back to tell us if this did work.
'Course, no one suggested another obvious solution: buy a workstand and service your bike right-side up.
I think the issue is that when you don't pedal, the derailleur gets jammed by the chain so it ends up stuck between gears and continuously drawing power (which is not only bad for the battery but must be terrible for the stepper motor and various mechanical bits like pivots). Pedaling allows the chain to shift, both clearing the jam and indicating that the RD was stuck in a bad state. I suspect that there's enough slop in most chains that you could get away with one shift on the RD, but if a button is stuck it will try to do multiple shifts until it jams.