The phenomenon is called "bearing fluting" and is similar to EDM (Electric Discharge Machining). A voltage potential builds up between the rotor and the frame and the current flows across the bearing races. But because the bearings are rolling around in the race, the discharge is not continuous so it builds up until it is high enough to "jump the gap" between the bearing and the race. As time goes on, the rate of build-up and discharge becomes repeatable and every time it happens, the race is damaged a tiny bit. But eventually, the damaged areas help facilitate the process so it gets worse and worse.
Reactors and such don't really help much. The true fix is what is called a "grounding bushing" on the motor shaft. There are a few available, but the most widely used one is from a company called Aegis.
http://www.est-aegis.com/
