They used the Heat's own medicine against them. Push the break, attack the defense prior to them setting, and as a result they drew fouls and got a couple of transition 3's.
It also doesn't help when the off-ball movement was not as frequent as it had been in the previous game. I cannot understand why in one game, their will be excellent movement and the next close to none.
What I can somewhat attribute that to is the inconsistency at the 1. Chalmers was over-aggressive when the 1st option was taken away and he completely abandoned the offense on more than a few occasions. The Spurs picked up pressure on the inbound and forced the Heat to use clock (same thing the Heat did vs Indy/Chicago) and weren't able to handle the pressure. They essentially forced the Heat into moving too quick and sloppy on offense. If you remember, Spo wanted them to be more decisive with the offense, and rightfully so. Bosh needs to get more touches(while being assertive), he looked good in the post and always has.
Defensively, the inability of Spoelstra to adjust to Parker iso'ing and LeBron to exploit Diaw was the nail in the coffin. The PnR did not kill the Heat, out of rhythm jumpers and lack of effort/focus did.
Miami performs well when they go small with at least 1 shot blocker on the court, I have yet to see Spoelstra break that out this series in extended minutes.
I'd be in panic mode had the Heat not gotten any transition buckets. Game 6 I expect them to be sharper.