Danny is a ridiculous Spurs hater, but putting that aside the Spurs fell off for a couple reasons.
The main one is that they were playing a run-and-gun offensive style that is great for the regular season, but the lack of defense was killer in the playoffs. When the shots aren't going in, there is nothing to fall back on... and against playoff teams there will be times when the shots aren't falling.
The second is that Memphis plays great, tough defense and fundamentally sound basketball, which translates well to the playoffs. That's why a lot of us, myself included, were praying for New Orleans (or anyone but Memphis) in the first round.
Third was that the Spurs got away from what got them the 1-seed in the first place. They went to a grind it out offense and a small rotation when they had been throwing the whole bench out and moving the ball like crazy... in other words, they played Memphis' game. Yet the Spurs still didn't play defense.
A distant fourth is that switching 2-3 shots here and there from makes to misses and they would be tied or ahead in the series. Those first two losses were close. It would still be ugly, but Memphis had to hit gamewinning threes to win the first two.