It's just wear and tear, it happens over time, contstant on/off pressure wears it down.
When I fitted my new one, I filed the lug to make sure it had a 90 degree angle against the clip, I also ruffed up the clip where it comes into contact with the lug, if you know what I mean. I don't like the design to start with, It would be better with a hose clamp IMO.