Cinema Blend posted an article that appears to reveal a little more plot for Thor: Ragnarok, specifically how our favorite jade giant ended up on Sakaar. From the article, it suggests that Valkyrie is a galactic “scrapper” and captured Hulk in a similar manner as depicted she captured Thor in the first preview. How did she keep the Hulk subdued (we don’t see the obedience disk)? Did she catch Banner or Hulk? Hopefully we won’t find out until November (I hope they don’t sneak much more) when the movie hits the theaters! The anticipation continues to build!
an INCREDIBLE HULK fan web site 
I would like to think Hulk somehow got transported off the Quinjet (since it did crash in the ocean) and like the comic when he passed through that portal it weakened him. He reverted to Banner and he was captured then. Somehow he was angered and there you go…a hulk out. Then someone gave the Hulk a pretty good deal…or threatened him in such a way to keep him fighting under their control.
If my portal theory is correct maybe it also weakens Thor enough so he can be caught too.