It's because of manual operation of the mirror by the owner by their hand or kids messing about. If the power mirror is manually moved (ie. Accidentally pushed) it is possible for the motor mechanism to become out of synchronisation resulting in incorrect operation of the folding mirror. To resynchronise the mirror, cycle it 3 to 4 times and then it should function normally. If after 4 cycles it does not correct itself replace the mirror as it has a fault.