DIAGNOSIS Water or condensation inside the handle is triggering the capacitive touch power button. This happens if the handle is stored lying down or in a very humid bathroom.
FIX Store the handle upright, not lying on its side in the charging cup. Ensure the button area is completely dry before storage. Leave it off the charger in a dry location for a night to see if it stops.
IF THAT DIDN'T WORK The touch sensor may be faulty. Try disabling the feature: in the Oral-B app, go to Settings > Brush Settings and turn off 'Easy Start' (which allows tapping the handle to turn on).
HEADS UP If it's constantly turning on while on the charger, unplug the charger from the wall. A faulty charger can send erratic signals that wake the brush up.