That's because environmentalists and feminists (and vegetarians) tend to be over-bearing, hypocritical, and unwilling to take criticism.
Mind you, not all of them are like that, I've met many vegetarians and environmentalists in my life that were calm intellectual people that I genuinely liked. I've never actually met a likeable feminist (although i'm sure they exist).
Another study, featuring 17 male and 45 female undergraduates, confirmed the pervasiveness of those stereotypes.
Not much of a study
For someone to self identify as a feminist...