One of several not unlikely scenarios: Cruz wins Texas and maybe a few others. Rubio loses Florida and every other. At that point, Rubio exits and endorses Cruz. Trump would then be in a world of hurt. Granted he could still win since by then many pro-Cruz states would have already happened. But Cruz has a strong delegate game. He would pick up more delegates in the races he takes second than Trump would in the races he takes second.

So much of this boils down to Cruz's Super Tuesday strategy working: winning Texas by enough that he gets a load of delegates, and having a strong delegate game in Georgia even through a non-first finish. He's not playing to win that many states. He's only going to Oklahoma and Tennessee once each between now and the election. Everything else is Texas and Georgia.