William,
I experienced the same type of problem on the front calipers of my car. I bought MB kits and repaired them. I found it to be a long painstaking job if you try to do it precisely. If I have caliper problems again, I will probably go to rebuilts. Come to think of it, I don't believe rebuilts were available when I had my problem.

As to what causes it, I don't know for sure, but I suspect that if you don't follow the MB procedure exactly and pull the pads with an impact tool and then press the caliper pistons back with their special tool, you may get the piston "cocked" in the cylinder and it then won't retract when you release the pressure on your brakes. It is like driving with the brakes on all the time.

Another possibility: there is a small rubber rib on the edge of the piston seal that allows the piston to draw back slightly.....it may just wear out.

Hope this helps you somewhat. Good luck.

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Ted
1979 240D
160,000 miles