Simple, MS wants to bury the game as much as possible and keep all attention on the X1. Since we've heard no figures, I'd take it the game didn't quite do quite as well as they expected (now that's not saying it didn't do great, just not as good), so they push the 360 back another 2 weeks in order to keep attention on X1.
The big thing is, after Titanfall, MS will fall into irrelevence for a long while. Their next major release would Halo 5 or Quantum Break (though that doesn't really feel like a system seller, it'll definitely get them buzz). Alternatively, a price cut could bring back attention as well. With Sony pushing ahead with VR, having marketing for two of the biggest (if not biggest) games of the year (Destiny and WatchDogts), and finally opening up about their first party development (hopefully at E3), it's going to be a very tough road.
I'd say I'm a conspiracy nut, except that MS has given us plenty of reason to doubt them, I don't see why it'd be different now. Why anyone still trusts a word they say, I've no clue.