I just checked and there's been no changes to the team that just passed me. The team in third did make some changes, but has only gained a couple of games. I checked and saw that I was even against the (formerly) #2 team in my division and had a winning record against the #3 and #4 teams. Now I can't beat any of them.
I was in first place with as much as a 9 game lead. My winning % for the first 118 games was .632. Since then, I've been 13-20 (.393 win %). Why in the world would a team just flop that badly? It always seems to happen around the same time. My team may have been playing a little above it's head, based on James' Pythagorean theorem, but nothing warrants a slump like this.
After three seasons, I'm starting to think this is something that is something that might be in the software, maybe in order to maintain interest.