He is a Dem plant and will throw the election so Hillary can win.
I'm a bigtime anti-conspiracy theorist but this theory holds water.
There is no way he wants to be president. Have you seen how much these guys age while in office? Trump has a sweet life right now so why would he want to trade it or shorten it? I think this plan was hatched after Hillary lost to O in 08'. Hillary knew she would only have one more shot and who better than her friend, donor and fellow NYer to run interference to ensure her bid for the presidency? What's in it for Trump you say? Can you imagine the debt owed to Trump after he gifts her the White House? It's almost scary how easily the GOP base has been played by their arch nemisis in the Clintons. Crafty bastards they are.
Plus, even if Cruz somehow wrangles the nomination, Trump will run third party and still hand the White House to Clinton. There is no scenario where Hillary cannot win.