They are probably tired, they have to give the day off to some players.
Their bench players were much more consistent last regular season. Not so much this time around. Coupled with Wade's relative inconsistency (also more pronounced this year compared to last) and Lebron is shouldering a lot more of the load over what feels like about the same kind of minutes.
They're still the odds-on favorite, no doubt...but they're not coasting right now. Some of their guys just aren't playing well.
Even with Miami's struggles they still have what the 3rd or 4th best record? they are coasting at times but are still come end of the season have a top 3 seed most likely. They have sucked bad on the road but you know come playoff time winning on the road won't be an issue for them.