1 of 2 things:
1. starter sprag failed (one way clutch)
2. flywheel has come loose from crankshaft
the "early" engines do not have a torque limiting function to the starter system, the gear system that connects the starter motor to the sprag is solid, no clacky clacky.
Given the nature of the starter and how its configured, I cannot understand how the starter would rotate in the opposite direction if the terminal where the cable attaches becomes electrically connected with the case of the starter, and when normally mounted to the engine effectively connected to the negative side of the battery..... I call BS.