Well the Yao Ming injury happened, which forced Houston into a smaller lineup and totally changed their style of play. They also had the home crowd and played extremely motivated, worried about the loss of Yao Ming. Conversely, the Lakers seemed to think the series was over (it is) and didn't come out with any passion and intensity.
So those two factors: the energy and effort disparity, plus Houston playing a style that was completely different and unexpected from what the Lakers had seen and prepared for, combined to make them lose the game.
Also, games happen quickly. That was one thing that I noticed; there aren't chances for many major in-game adjustments. If the other team is doing something totally different than expected it can really throw a team off. The Lakers were ready to play a methodical, slowed-down game tempo; and that just never happened. Houston was ready to play fast, had early success, and played even faster, urged on by the home crowd.
Streak update: record is 3-3. I lost my third pick, Orlando to beat Boston, and then lost two random picks trying to get a win so I could start a new streak. Finally got it on the Dodgers-Phillies game today. And I don't think its possible to pick 27 baseball games in a row.