I have heard of horses going over backwards as a freak accident (spooked, reared, stumbled, something like that). But a chronic rearer that has gone over is a bunch of red flags to me.
God only knows how he managed it but he managed to get his balance again and didnt go over but he was so close it was scarey. He was a very very powerful and well balanced horse which is probably what saved him from going over.
He scared himself that day as well.
I don't count that as a purposefuly dangerous rear, yes it ended up being dangerous because of the whole situation but he had absolutly no intention of going over backwards.