I took my 5 year old stallion in to be gelded last May. He was gelded due to lack of use, not because he was a behaviour problem. He was always a total love bug, eager for hugs and kisses and even around a mare in heat, not hard to handle at all, though he was happy to "oblige" if we wanted him to. He had been hauled in the trailer with mares right next to him, no problems, no Vicks needed, nothing.
WELL! When we picked him up after his surgery, he came out of the stall all puffed up and studly, loaded him in the trailer and then put a couple of out of season mares on and he proceeded to just about tear the trailer apart. Brought him home, put him in a nice big round pen so he could move around and keep the swelling down, and he basically gave everyone the finger when we'd go out to feed or water. Nope, no kisses, forget it, don't wanna talk to ya. We kept him separated for a month, then turned him out with his girls. He was hilarious. He walked that pasture with his ears pinned for a month. Today, he's right back to being a big old love bug and wants ALL the attention.
My guess, and that's all it is, is that maybe the get a 'rush' of hormones when the surgery is done and then couple that with being sore/swollen and generally unhappy with life (think appendectomy pain in a human), and I think they just let us know that right now, life isn't great and they aren't liking it much. Once everything settles down and quits being sore, they come back to normal.