Just so you know, there is nothing that changes in the engine if one team is strong in the first half that would give the other team any advantage to catch up and win. To be honest I intended to in the original design document, but found that it happened naturally frequently enough so threw that idea out.
If you give up a big lead in the second half, there are a few possible reasons:
1) You go into "slow down" mode (per your misc gameplan settings) and your opponent catches up (but even here, once you are back within the threshold you'll come out of "slow down" mode
2) The dice rolls suddenly start going against you instead of for you
3) Your opponent goes into "hurry up" mode and is actually more effective passing the ball than running (and I know that passing is currently too strong, a point of emphasis for the next version)
I think #2 is the most likely, especially in the playoffs - a team isn't going to be in the playoffs if they aren't reasonably good, you should be fairly well-matched and by the very nature it should be tough to blow a team out in the playoffs.