Back when I did a lot of field work with elk biologists, I learned this does happen from time to time. Bulls will get locked up and break a pedicle while trying to separate. The forces they exert during a sparring match are all forward and down but as they realize they are stuck they panic and try to pull away while twisting and bucking. It's clear from the photo that in this case, the skull broke below the pedicle, which would have caused certain death. In any case, it's an interesting find for sure. Thanks for sharing!