The term eco warrior, in the UK at least, long predates social justice warrior. As the peer reply says, it's long been applied to members of Greenpeace and I think I first heard the term in the late 80s or 90s. As they said, maybe it was because of their ship Rainbow Warrior which was sunk by the French government in 1985 and prompted Greenpeace to continue the name with future boats.

I don't think it particularly had a negative connotation until recently though, to me at least it was always just someone who had strong opinions about protecting the environment, and Greenpeace always had quite a lot of support from the general population and they weren't actively disrupting the lives of ordinary citizens in the way Just Stop Oil do for instance.

I think you have the wrong end of this stick. See the Greenpeace vessel Rainbow Warrior for an example. There have been several iterations of this ship name since the first was bombed by the French secret service in 1985.