In the description I read, the narrowing semi-circle in which Hannibal was forcing the Romans into became so tight, either because of his commanding his troops to push in on the sides, or because the Romans were being overeager, and trying to force through the enemy line. This was only compounded by the veteran flanking troops slamming into the backside. It's my belief that they weren't able to escape, because there weren't able to fight. They had been forced in so tight they scarcely raise their weapons or shields to fight.

Source: Mark Healy's "Cannae: Hannibal Smashes Rome's Army"