Could have been any one of several things, not the least likely culprit being dirt and grime. Over the years some people have had trouble letting shells slip and get under the carrier, and there is a "fix" for this on the 870. You cleared it one of the easier ways. I have never had a problem in over 45 years with multiple 1100s and 11-87s, but I didn't ride the short bus either.
What could have happened... did.