There will always be a reason - you just haven't found it yet.
The motors inside will very probably have brushes but are unlikely to be at fault.
I would guess the most common cause is when people cut the cable by accident. Then perhaps the switch interlock mechanism. If the brushes were failing you would likely get warning as sparks and smells from inside send you increasing signals before they fail completely.
Frequency of use, continuous duty cycle and load on the motor will all have some effect on useful life.
Peccavi
June 2009