Wow, that sounds really scary. Leashes and collars are so commonplace, it's easy to overlook their dangers. I learned the hard way never to tug on the leash if I'm not watching the dog: I tugged impatiently once when it was wrapped around her leg; she squealed but wasn't hurt. You hear of freak accidents, collars getting caught on things. A friend lost a dog who fell out of a pickup truck bed on too long a leash and strangled (a not uncommon nightmare).
Not pleasant to think about, but worth thinking about. Thanks for sharing this cautionary tale.
Hope Max is OK and has an uneventful rehab. Youth is on his side.