Of anything I've read on the subject - and that isn't much - this makes as much instinctive sense as anything that attempts to describe what's going on:

It's a small sample size of one game, but it was clear Thursday night that Miami's Big 3 don't play well together. When James, Bosh and Wade were on the floor at the same time in Game 5, Miami was outscored by 14 points. When James was on the floor with Bosh but without Wade, Miami played Dallas virtually even... In the three minutes where LeBron was on the the floor himself without the other pair, Miami actually outscored Dallas by 5.

via ESPN.

I've seen this team dynamic before...