Vibrations and bumps in the road are two reasons. It is fairly common when I drive my old pickup truck but nothing when I use my new car. It cannot be adjusted.
I've mentioned this before but I have a riding lawnmower and it takes two hours to mow. The Apple Watch would register about 30 minutes of exercise and about a mile or so of steps just from the bumps and vibrations. My old Up24 was more like 5 miles and a borrowed Fitbit was almost 2 miles of steps.
Not sure how to avoid other than to take off/shut off or maybe really good shocks.