I have the same problem, but the other way round. I relegated from division 1 even though I was a good 9-10 points clear 24 hours ago.
There's 2 issues to consider:
1. the promotion/relegation limits fluctuate, meaning they can go up or down. I assume its to ensure the top/bottom X percent of the teams promote/relegate. If you were just 1 or 2 points clear of the limit then you might have been unlucky as the limit might have caught up to you during the night.
2. The ranking doesn't update immediately. I don't know if this explains why I relegated (I'm 99% sure the relegation limit didn't go up 10 points overnight) but its a plausible theory. I played my last match and decided to call it a day. I checked the division rankings but it hadn't been updated immediately. So I go back to main menu and start the MLO again. It said I had something like 515 points and that relegation was something like 505, YET even though I was 10 points ahead it still said I was in a relegation spot. I thought whatever, it will update soon so I quit and turned off my xbox.
My guess is you might have to stay in the MLO menu for the division ranking to update.