Highish speed and bumpy = autoshift?
So I was pedaling along with minimal effort at probably about 80 or 85 rpm going about 20 mph and the path got bumpy and wham it my rear derauler shifted into a smaller cog. Happened twice. Is that normal for bumpy areas? I'm running all shimano 105 components except for the rear derauler which is a cheesy plastic suntour model. I'll allow that this may also be the culprit.
Suntour and Shimano drivetrain components generally don't work very well together. Install a Shimano derailleur from the same era as the shifters.