Check out the "bezerk mode" thread and you'll see a bunch of us talking about it in there. There's no rhyme or reason to why it happens, and there's no 100% foolproof way to mitigate the issue. The developers won't even address it on their forums.
What I've found works the best for mitigating the issue:
1. Hard power off my Xbox before playing NHL. If I play any other game before I decide I'm playing NHL, hard power off. If I'm guessing I'll be playing NHL the next time I turn my Xbox on, hard power off.
2. Once it does happen, I will finish the game (I use Season Mode btw) then I will go switch a slider then switch it back just so I am given the ability to save my "change". I save my change, go back to the Rules menu, scroll through my sliders till I hit the one I want to play with. I then Save and exit the mode, then I hard power off.
Yes, this is the trouble I (and many others) go through just to play this game, yet the developers won't even acknowledge the issue. This is the reason I believe there's some sort of artificial coding going on in these games. If there wasn't, how would stuff like this ever happen? You can't have completely different game speeds on the same exact slider set if there isn't something else manipulating the game on the backend. It's so frustrating, I don't get why this issue can't be solved...or even acknowledged.