Hulk of a Different Color - S2-E1
Other mistake: When Betty is watching surveillance footage of her father attacking the gray Hulk, the footage is same clips from earlier in the episode, camera angles, shot changes and all. Not to mention it's extremely unlikely she would happen to have surveillance cameras in the exact random area of the desert where the attack happened.
Answer: The Hulk is a giant brute who smashes everything in his path when he is angry. In Ang Lee's "Hulk," General Ross, played by Sam Elliot, explains it to his daughter.