The problem is that high voltage spikes are arcing across the air gap in the bearings and leaving a little burn each time. It gradually builds up and leads to bearing failure. Two areas of solution would be to prevent the voltage spikes or to better ground the motor shaft. An electronic choke might be used to prevent the spikes. A grounding strap or brushes might be used to ground the motor shaft. I've heard of some motor manufacturers
doing the opposite. Instead of making a better ground for the shaft, they better insulate it from ground by using ceramic balls in the bearings. However, that's not a solution a user can easily do himself in the field, so I'd stick with the choke or ground strap.