Originally Posted by
Andrew R Stewart
No brand or model, no location or style of shifter.
If the rear der's low gear limit screw if right on the cusp of enough travel to get into low gear the slightest cage movement back (to the high gear end) can cause the chain to try to shift.
That was my first thought and have seen it often. Since it is easy I would check that first. The same could happen if the derailleur hanger or derailleur is bent and the chain is again "right on the cusp of enough travel to get into low gear" and drops back a gear when pressure on the shifter is released.