Continuity mistake: After the Hulk breaks the wall to escape at the end of his second transformation, he is wearing black pants instead of his usual beige pants.
Revealing mistake: When the Hulk is lifting his left foot at the beginning of his first transformation to break free from the "leg shackle", there is a close shot where we should see his toes. But we cannot see them because he is wearing some green pantyhose with brown soles to protect his feet.
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.