I have seen rattlesnakes become incredibly aggressive. I've only seen it in the desert on very hot days; the local wisdom is that when they get too hot they go crazy. I have seen a rattlesnake viciously pursuing and attacking an innocent pickup truck. Whether that's the reason I don't know, but I've never heard of something like that happening in more temperate regions.

You're too big to eat, so if you don't startle them they'll almost always choose to leave.