Ever had self adjusting brakes overtighten?

Several other things come to mind;
1) the wheel cylinder pistons could be sticking
2) the fluid may not be returning to the M/C
3) the fluid is boiling.

check that the shoes are properly parked on the anchors and that they return smoothly after application.
If you have water in the w/c and it boils, the resulting gas pressure could apply the brake, if it can't blow the fluid back into the M/C; ie if the compensating port is restricted ,plugged, or blocked by the piston.