What Does It Mean to Have Your Back Leg Collapse?

Ok I’ve noticed that a hand full of people have said their back leg collapses but I thought that your back leg naturally has a bend to it when your going toward homeplate? And I’ve seen many pros who almost scrape their knee on the ground because their leg bends so far when they are driving toward home. So is this what people mean when they say their back leg collapses or no?