On the motor it's self, there is a white plastic arm. The arm rotates, in the groove, or slot. I am guessing that the tip of the white plastic arm has gotten worn down. That's my guess. If that is the case, the tip needs to big enough that it will not move out of the slot. If that's the case...
I am guessing you are using a sportsman. Have you tried to find the problem? Have you overloaded your turning racks? It would be great if you could take some pictures of the tuner after it has overturned, and we could help you a lot more.