Its all to do with the way the bites ware .Put another way its lack of maintenance . This is why gunsmiths recommend periodic servicing of guns , frequently being able to pick up these potential problems before they cause trouble .Fairly simple job building up the bolt re-cutting the bearing faces and checking clearances between barrel wings and action ,but people do put off these things .

Bottom biting O/U's all makes can be prone to this , although I have known centre biting guns like Beretta's, have a similar problem due to looseness and excessive ware on the bite .