TL;DR The current best hypothesis:

>They figured it was more probable that young male mammoths were much more likely to travel solo, away from the wisdom and protection of matriarchal herds, similar to the way elephant societies function today. In other words, these male mammoths—young, reckless, foolhardy—were just more likely to get into some kind of trouble and die, from getting stuck in a pit to running afoul of hunting humans. Luckily for paleontologists, some of these death sites—bogs, crevices, lakes—are pretty good at preserving remains. “They were more likely to do silly things, like die in tar pits,” Gower says.