I never heard of that just happening for no apparent reason. I would suspect there was somehow some history of abuse, although I cannot think of an example.
I happen to have a run-of-the-mill Reese 16k hitch. I separate the two halves, turn the top half over, clean, inspect and grease it every year. It always amazes me how little metal is actually there considering the job it is required to do.
If I had it to do over, I think I would have bought a higher rated hitch just for the peace of mind of knowing I had more cushion to work with (15k trailer and 16k hitch).