Refusing to gate, to me, is a huge issue, and it tells me that there is an underlying issue that needs to be addressed. Hock problems, back problems, saddle fit problems, and a hole in training can all manifest themselves as such. The rearing at the trailer (when tacking up, I would guess?) tells me that you might want to look into back soreness and/or saddle fit.
When you're in that situation at the gate where he's refusing to go in, you might want to look at exactly what's going on and break it down. Leave the over-under out of it, and think about what the underlying problem is. What kind of bit do you run in? Do you use a tiedown? Is the tiedown adjusted correctly? Tiedowns can restrict head movement, so if a horse goes up, they don't have the neck to rely on, which can lead to the horse flipping. So many factors could have played into what happened, I strongly suggest you take a really good hard look at everything before you go run again.