L50 repeatedly extends and retracts arm??

Why does my L50 repeatedly extend and retract it's rotating cleaning arm while charging in the base? It does this every two or three minutes, over and over and over while it is parked in the base. Extends for about two seconds, then retracts. A few minutes later, same thing. Again and again and again... Surely this will wear it out eventually! Also, it's mildly annoying to hear the noise every few minutes - although that is not my primary concern.