I experienced this at my old apartment and I always wondered what the reason
for it was.

The outlet in my bathroom was a GFI and I used to do light metal polishing
over the sink with my Dremel. Anyway, after using the Dremel for a bit the
tool would start to bog down so I'd back off the polishing head and as soon
as pressure was released the polishing wheel would spin up dramatically
actually overspeeding then the GFI would trip.

I can see the Dremel slowing down, I can see it rev'ing back to its original
speed once the load was removed, but why would it overspeed and why would
that trip the GFI?