Something was up with my SLX back brake last night, lever felt stiff and crunchy.

Had a look at the caliper today and the one pad (superstar) had stuck itself to the inner bit of the piston, which had worked loose from the outer bit of the piston- which I guess was causing the crunchy stiffness. Had to actually pry them apart, cleaned up the pad backs and put em back in, working fine now.

Why did the piston inner come away from the outer? is it likely to be effed, and why are the superstar pads sticking to the piston?

octopus disclaimer- actual pads are still working fine after many miles of wet and dry rides with the braking surface intact, installed with use of vernier rocks etc etc.