It is very difficult to tell what happened for sure.
I suspect that the gear teeth broke first. The were in the area of the gear that would be engaged when the butterfly was full open.
Let's postulate that the butterfly froze up in the open position. Maybe due to winter ice, and then the cold brittle plastic teeth snapped off as the motor tried to close it.
Once the teeth broke, the flap is no longer pinned hard in place; it can move a tad. The position feedback sees the flap out of position and tries to move it back into position, but the teeth are broken off, so the motor runs continuously.
It is only made for intermittent duty, so the motor overheats and fails.
It is as good a guess as any.
With the power off of a healthy PD ASV, you can reach in and move the flap with a fingertip on the butterfly plate. When you apply power it moves back open.
When it has power, you can move it about 1 degree out of position, then the motor kicks in and re-positions it full open. I would only try this when the plastic was warm.