The same thing that happens to most other teams that have a sensational year, they fall back into the pack
Sustaining success is hard, maybe players (and coaches) take success for granted. Once they reach that pinnacle, they think they're just going to stay there. Or maybe other teams figure them out and they don't make enough changes.
Almost every NFL team regresses after a great season. Many MLB teams do as well. There's no regression in the star driven NBA, but that's the only sport like that.