Usually stress on the hose that it connects to. Hard to say, it's plastic and plastic can fail? We see it happen a couple times a year. Usually older spa's. If the thread is still in there you can take a flathead screwdriver and heat it up with a torch, not mega red hot though. While the end is hot stick it in the the threads so it kind of melts in, let cool the turn slowly counter clockwise while applying a little pressure. Just make sure the threads are still in there! You don't want to melt the threads that are meant to be there on the pump head. Thats a bad thing!
