The Illuminati shot him into space. There was a malfunction, and he ended up on Sakaar instead of the paradise planet they meant to send him to.
I love how they kind of hinted at the fact that a "paradise planet' for Hulk was one barren of intelligent life but covered in big game for him to hunt and hulk murder lol
Like you know Reed and maybe even Tony had MULTIPLE options of planets and there were a bunch of green and lush planets with like bunnies and shit and they were like
"nah... nope. No. He'll get bored and turn the planet into a spaceship and come back and kill us."
Because somewhere deep inside they both know that there is a Banner in there and he'd figure it out... so they better keep him busy!
Love Planet Hulk. Adore WWH. As long as they don't fuck it sideways in the ear i'll be ok with however they end up getting there. My bet is on Loki. I can see one of the big climax scenes now - Hulk and Thor walking into an asgard palace after laying serious hurt on misguided guards and enemies alike... Thor see's Odin and Hulk snarls and starts walking menacingly towards him...
Thor: Hulk! No my friend stay your hand! That is my Odin the All-Father!!
Hulk sniffs: This is a puny god...
much smashing.