This thing right here:
View attachment 112539
The drum key screw on top should keep it locked in place. It might not be tight enough. The screw end might be rounded. The shaft might be rounded. There might be some sort of lubricant in there causing the parts to slip.
Take out the screw on the top. The spring perch (the metal part) should slide right off. You can now inspect the screw and shaft and their contact points. Check for lube. Clean thoroughly if you find some. I have a WD40 horror story I'd rather not get into. Stuff happens.
If you cant locate the issue, replicate the problem SLOWLY and watch how the parts slip. Does the screw move? Is there a crack you couldn't otherwise see? Look for anything that seems weird. You should be able to spot the failure as soon as the perch breaks from its normal position. Good luck.