Self adjusters keep overtighten

Here are the photos. As you can see some material on the secondary shoe had come off. This was new to me. Did not look like that when I backed of the adjuster the time before today. What would cause this? The heat from the sticking break? I always managed to push the car even when they were sticking. So it has never locked up or anything and I always cought it early. Now unfortunately I did not have a helper tonight so I could never apply the brakes or parking brake and watch. But maybe the photos could tell something.