Get behind, and you must resort to high risk/high reward game play in order to make up ground. Defenses start going for picks and fumbles and not tackles. Offenses go for a higher percentage passing plays, which marks it easier for the other defense to read, etc.
So, a first half only advantage could cause the second half blowout.
Yeah, that was it. The Colts SUCK. The only player worth his salt is Luck.