Probably due to a bumpy road or bad suspension. I recommend disabling the Move IQ and Auto Activity Start options, and only starting your walks and other activities manually. You can also start the Driving activity manually, when travelling in a car. If your watch does not have this activity profile, then you could use for example the Cycling (using GPS) or Indoor Cycling (w/o GPS hence using less battery), and then changing the activity type to Driving later in Garmin Connect.
