I got reared from a similar angle back in April:
I can roll with the logic that polearms rear horses from the side (realistically, it's not like a spear would stop a horse from the front either), but for the sake of balance, I'd probably put the rearing cone to something less than halfway around the horse's body (from the front). Maybe a spear attack to the horse's head could rear it from any angle? All depending on the speed it's going.
I'm not sure if this is true, but if you need to have your feet on the ground to rear a horse, then that's just a pointless nuisance.
In any case, I've been getting reared much more frequently in 1.5, and I don't get frustrated by it. I think to myself: "Fair enough, I was too careless."