Where's all the auto electricians?
I'd suggest that the alternator is toast. I'd also reckon that the regulator
(inbuilt or external?) was sticky & feeding power "backwards" & in effect
turning the thing into an electric motor? I bet you if you didn't have belts
on it (or wasn't direct drive?), it would have been spinning.
Either way I'd be getting it serviced. (or replaced now?)
BruceM