Since I actually went back and read the thread, the only other thing I have to throw in here, which seems to have been hashed a lot, is that this is primarily ping based. The other thing, something about a 60 degree angle. It's possible, but the animation doesn't always reliably portray where the actual hit is taking place. So, while a long spear for example appears to be hitting you in the side of the horse, it could actually be toward the front causing it to rear.
I play with 20 ping and attempt to rear cavalry every chance I get. I hear every so often people complaining that my weapon was nowhere near them. For me, it was a perfect rear, which leads me to believe it's more ping and perspective than anything.