I think your Head Coach is more likely to try and run the clock down in the second half if you're leading. So he calls 'safer' plays.
Not sure if it's intentional but you do see a similar thing in real life when teams suddenly flip a game on it's head after one play/incident sparks a change of momentum.
This though is one of the instances where Smirt and I agree, I think the reality (my opinion, pure speculation) is that the game has tripwires to keep things close - penalties, incompletions, turnovers. Whether that can be by-passed with rules is debatable, but there are also days when everything clicks and you run out easy winners.