I think they got cute and talked themselves into thinking they were about to catch us being overly aggressive or something.
It's also possible they had no faith in being able to pass-block long enough for someone to get open downfield. And even if they protected long enough to throw it downfield, had lost all hope of the receiver holding onto the deep pass even if it hit him right in the hands. So they thought, "hey, let's throw it short and get a convoy going. I'll bet on their defensive backs missing tackles and turning the ballcarrier loose sooner than I'll bet on our receivers holding onto a pass."
But I tend to think it's the first one.
Lots of offensive coaches outsmart themselves and make dumb playcalls because they have this idea that the defense won't see it coming and they'll show everyone their big brass balls.
When teams are in a time-crunch situation, it's why you see lots of teams throw the ball in the middle of the field instead of towards the sideline. Or why you see a ballcarrier turn upfield instead of heading for the sideline like he ought to. They know the defense is expecting the ball to go to the sideline, but they have this stupid fantasy that they'll do the unexpected (throw it down the middle, or turn upfield instead of heading straight to the sideline). They have this fantasy where they do some brave, ballsy thing that catches the defense by surprise and goes for a TD, and everyone hoists the playcaller or the QB or the ballcarrier (whoever made the dumb decision) up on their shoulders and start chanting their names, like the end of Rudy. It almost never plays out that way, though.