Continuity mistake: When the lab is set on fire and the Hulk jumps into the room to save Elana, the Hulk that jumps through is actually a stunt double. Noted because the Hulk that jumps in isn't that tall.
Continuity mistake: When David gets out of his car and looks at the flat tire, the hubcap is gone. When he goes to the front and jacks the car up, he uses the jack handle to remove the hubcap that wasn't there before.
Answer: The Hulk is accused of killing Dr. David Banner and Dr. Elenia Marks. After the lab explodes, McGee sees the Hulk emerge carrying the body of Dr. Marks, so he assumed the creature is guilty. It was McGee who caused the explosion. After following the trail of evidence of the Hulk to the lab, he hides when David and Elania arrive. In a darkened storage closet, he knocks over a bottle of flammable liquid. He confronts them, then is forced to leave by Banner, who says, "Don't make me angry, you wouldn't like me when I'm angry." When Banner reenters the lab, it explodes, caused by the Hulk when it went on a rampage trashing the lab, causing many electrical systems to catch fire, thus igniting the flammable liquid.