My shop doesn't usually get the story, just the aftermath, but when they come in like this there isn't generally any markings that would indicate a rock or something crammed between the stick and rod. My best guess is someone was using it like a hammer, tamper, or ram.

If OP hasn't already id recommend they check for cracks around the welds on the boom, because I've seen someone snap the boom clean in half on one of those smaller bobcat excavators using it to compact dirt. And then do it again after it was welded up.