What caused inboard front disc brake pad to score the rotor?

Chrysler large single piston front calipers.

I'm trying to figure out why the inboard pad kicked its shim halfway out from behind the pad (until the shim hit something and could go no further) and scored the inside face of the rotor.

Did the shim let go first and cause the pad to do weird things? Or did the piston fail to retract correctly and make the shim jump out of place?

How exactly would you check to see if the piston is retracting correctly on single-piston calipers?

Thanks very much, everyone!