I agree with what Steve Kerr said last night - he said he talked with Flip Saunders about what was happening with Dallas and Flip said the same thing happened to the Pistons last year.

They were on such a roll that they didn't have to play any meaningful games for about the last 6 weeks of the regular season. They lost their hunger and their edge and it cost both the Pistons last year and the Mavs this year.
I think they wore themselves out trying to win 70 games by playing starters for too many minutes. Notice that their shots started coming up short late in games. They are tired.