Not entirely sure on the physics of it, but I would imagine as the recoil spring weakens, slide velocity increases and the barrel lug slams into the slide stop with more force. When I talked to a CZ-USA rep about it regarding an issue I had (see below), he mentioned that the slide stops are designed so that they break when the recoil spring is totally shot as a way to protect the frame from damage, this is more relevant on aluminum alloy frame guns but can still occur steel frame guns, per the rep. Just another reason to be zealous about following the recoil spring replacement recommendations.
In competition circles people typically run lighter recoil springs that (I believe) lose length faster (my conclusion from a data set I saw on loss of spring length over time across various spring weights in a 1911) so broken slide stops are more common in that setting if people aren't regimented about recoil spring replacement. That, plus higher round counts overall in competition guns and more parts wear. From what I have seen and heard, very serious competition shooters usually have separate, identically set-up, training and major match guns, no matter the pistol platform to mitigate parts and spring wear issues.
Interestingly enough, I had a slide stop break on a P01 Omega where the lever portion sheared off but the post remained in the gun. I actually finished the course of fire and started another one (20-30 rds with it broken) without realizing it had sheared off, the gun functioned fine but the slide did not lock back, obviously. In that instance I traced it back to putting a worn-out recoil spring back in the gun; I had kept a worn out one around for reference and grabbed it in a rush and installed it instead of new one, totally ignoring the labeling on the bad spring. Lesson learned though, recoil springs over the round count go straight in the trash now. Another slide stop break I had was in a P01 steel frame, where the slide stop (different design from the omega one) broke in the same place and also continued functioning until I noticed (5-10 rds). That I attributed to having a borderline recoil spring and likely inconsistent round tracking; that slide stop had also been re-coated with a PVDF coating which also may have contributed to earlier breakage (pure speculation there). Both guns were running stock weight recoil springs.
So what I have learned is that it pays to keep a solid spreadsheet on the rd count on your recoil spring and replace it in advance of the recommended round count. Not a bad idea to have spare slide stops too.
