If you set your overall team game plan to be able to change defensive positioning whether it's "if losing" or anything other then "never", which it sounds like you did. Then if your team is in that designated situation, the sim will automatically adjust up to + or - 2 defensive setting, I believe.
I'm also pretty sure the basis of the change is how many 3's your opponent took in the first half.
This makes sense logically why the game would do this, and doesn't need to be changed. if you don't want your team to change at halftime, set it to never, and the other end of game adjustment should still work. But if a coach plays a team that shoots 30 3's a game, and the opponent knows that coach will play a + defense and adjusts by telling his players not to shoot 3's, you'd want your team to adjust to that at half time.
That's why the sim does it. If it didn't, there would be a lot of complaining from coaches that got out game-planned and couldn't do anything about it. Half time adjustments are pretty realistic.