This is a classic "overrun" starter that we see with diesel engines a lot. What was happening is you had a poor connect for some time, this caused the voltage delivered to the starter to drop. Because of the lower voltage the amperage had to increase in order to get the required wattage to turn the starter motor. The higher amperage turns the contacts in the solenoid from just working as a switch into a "spot welder' which drops the voltage even more as the contacts get pitted. At this point one of two things happened; the starter switch went bad and stayed in the start position while in the "on mode" or the solenoid contacts got a good "spot weld" and remained closed. With the 10 to 1 ratio of ring gear to pinion gear this means that the starter was turning at least 10 times the engine speed. I would expect the starter is damaged beyond repair. It is very important to check the voltage at the starter during cranking to test the wiring, even better is to do a voltage drop test on both sides of the circuit, positive and ground and make sure the total drop is less then .5V.

Corbett
Regional Manager for the largest starter and alternator company in the world
This is not offical company business