Revealing mistake: During the first transformation, just before the Hulk breaks the wall, we can see the green shoes he is wearing to protect his feet.
Continuity mistake: While Dr. Morrow is saying, "I did some preliminary tests on David. The limbic system is definitely affected," he is holding a folder containing patient information and a picture. In the next shot the folder contains a different picture with no time to have turned any pages.
Continuity mistake: When David is on the hospital bed before his first transformation, he is wearing beige pants. During his transformation he is wearing light blue pants, but when fully transformed into the Hulk he is wearing beige pants again.
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.
The Hulk didn't cause the explosion. The liquid mixed with another chemical. While David and Jack were both outside, Elaina heard a sizzling noise, and when she saw smoke coming from the closet Jack was found hiding in, she called out to David. That's when the explosion happened.