I expect they anticipated the Dolphins would throw the ball as far as they could and hope for either a lucky catch or a pass interference penalty. Plays like that often end up in a "jump ball" situation, with a bunch of guys from both teams waiting for the pass, jumping for it, and hoping their team would be the one to come down with it. In a situation like that, who better to have jumping up for the ball than the Gronk? He's one of the most sure-handed receivers, and he's also a huge strong man who would probably beat most receivers in a situation like that.
-k
They were way too far from the end zone for a Hail Mary pass....