She rode him again today. I changed the bit and let her ride him much more forward. The previous owner confirmed that he tend to do that a few years ago as well. Today went well and he jumped a full course for the first time in his life.
I'm going to begin with a simple sentence - nothing is ever the horses fault.
I say this because there is always a reason behind why horses do what they do. Rearing is the horses way of telling the rider he is unsure of walking forwards because of either fear, pain or disrespect for the rider or a misunderstanding of aids given by the rider.
Most of the time, I find that horses rear because they are attempting to escape the pressure in their mouth caused by the bit, or do not fully respect or understand the rider's cues.
The fact that your daughter's horse fell over backwards suggests that your daughter accidentally dragged him over backwards using too much of a strong contact. It would explain why he reared up in the first place, and then fell over backwards due to your daughter not giving the horse enough release in his mouth, causing him to become unbalanced and topple over.
I'd make sure that your daughter's reins are not too short, as this would pressurise the horse's jaw and cause him to rear as a result of him trying to escape the pressure. Signs that he is uncomfortable in his mouth are:
- opening his mouth/gaping
- chomping on the bit
- throwing his head + pinned ears
- swishing his tail (a sign of frustration)
From the information given I presume it is a bit issue, therefore that is what I have focused on, since this is the most likely cause of rearing. However, if your daughter's horse shows no signs of discomfort in the mouth, then it's more likely to be a respect issue. If so, move his feet to gain his respect through lunging, changes of direction, backing, yielding, etc. Basic groundwork and lots of desensitisation gains your horses respect and builds your relationship.
I really hope this helps! Remember, find the cause of the problem, go back to basics and work from there! :)
If you can catch him before he goes up wack him on top of his head with your crop. He might think there's something up there he's going to hit his head on and might stop. But I would really recommend consulting a professional trainer as well.