Why do some denialists claim to be environmentalists?
I'll admit it's a successful trick for increasing their annoyance quotient, but it's just weird. They really like their conversion stories as pseudo-arguments from authority: "the evidence for my crackpot position is so strong that it even overcame my initial bias, so come join me!"
It just makes them seem flighty to me, using a standard creationist trick that means nothing. Maybe for a few of them it's even a true statement and not a bald lie, but it's no more convincing to any reasonable person who's glanced at the science.