Some horses arch their back to then make it easier to strike out with their hind legs
My horse tucks his head in before and when he is bucking x
My pony arches her back before bucking
Every horse is different but I am riding a horse that bucks a lot in the canter so I would try to lick him/her head up and sit back
It really depends on the horse but my horse always does a really quick turn then starts bucking.