It's difficult to say for sure, but if you can rule out mechanical causes and from what you describe, I would first suspect the gyro is sending a hardover signal to the servo. That being the case: why? Has it had a low voltage induced shutdown and subsequent inflight reset or is it just defective? If it's low voltage, it could be the BEC being overburdened which could be due to, say, a defective servo pulling too much current. See where this is going, it could be anything. This is where it is really nice to have some spares to swap out electronics one at a time, starting with the best guess.

If the motor bearings are not the cause, they likely will be in a flight soon to come. Get them replaced. But I do not see how they would be involved in this hardover situation after the heli is down on the ground and motor off.