Thought I'd share the reason: Go back and watch the third quarter. There are two Spurs offensive plays for the bulk of it that weren't jumpers. Parker's and-1 against Bynum was the only one until Manu's cut if memory serves. When they hit the third or fourth jumper to extend the lead in the third, I actually turned to my wife and said "The Spurs are about to give up this big lead and lose the game".
The only reason they didn't lose the game is because the fool's gold feeling from hitting all those jumpers actually helped them play with energy on the defensive side, which isn't always the case. Once they missed a couple of them, the floodgates opened. They were able to get it back under control with some inside play and some good defense, and hit some more jumpers late to inflate the margin back, but the all-jumpshot mentality they came out with to start the second half could have been the killer.