The last trailer explained the time traveling rules for Hot Tub Time Machine. But this trailer explains the why these four pathetic dorks were sent back into time. The answer: To fix their collective futures. Spoilers below.
This trailer doesn't explain all the characters' future issues, but Craig Robinson reveals that his future wife, whom he can't cheat on, was cheating on him. Also it seems like John Cusack missed out on "the girl of his dreams," while Rob Corddry, well maybe he was just sent back to have a three-way and "make Hitler president."
Who wants to bet money that they don't actually change their lives along the lines of Back To The Future? Instead, they probably learn to appreciate what they already have. Lets hope not, because I personally always lived by Biff rules: Steal the Almanac, become a millionaire and kill the competition. It's much more fun that way.