There has been a rash of injuries. That can't explain a total meltdown. Zednik was missing for most of the early season tear, and lots of teams have had injury trouble without imploding. Partial explanation at best.
They were never that good to begin with. When the Habs were winning a lot of one-goal games, many said it would only be a matter of time before their good fortune would end and the team would fall back to Earth. I usually agree with these statistically-sound arguments. However, in this case I saw most of those close games and felt that Montreal often soundly outplayed their opposition, and it was the opposition who were fortunate to have lost by only a goal. Even when they started losing, I thought they still outplayed their opponent on several occasions. Let's call this another partial explanation.
This year's crackdown on obstruction is waning. I think the Habs were one of the teams that benefited most from the early zero-tolerance interpretation. The Habs are a good skating team; their weaknesses are size and physical play.
Everyone else has caught up. Every key player for the Habs spent the lockout playing high-level hockey. Perhaps all those players who didn't have finally shaken off their lockout rust and the Hab's advantage is now gone.
There - I think four partial/possible explanations add up to one big fat excuse.