Coco use to do that in his old stall ..stand up like that on one of the bottom boards, about scared me to death bc i thought he was going to slip and break his leg.
One time, my gelding Peanut was trying to "help" me by gettng his grain dish... he ended up hitting me in the head when he swung it and I got a black eye...hehe. No one believed me when I said my horse just wanted grain...lol.