[prewett]

My current theory is that it's a reaction to being so disgusted with the values of the outgroup that they reject everything that the outgroup supports. You can see the other version of this by people who want nothing do with Tesla/Musk because of his political views. Environmental activism, and, say, gender activism (the real hot-button one) have a similar flavor, so if one is disgusted by activism in general, or a specific activism, one might just reflexively reject the other. Just like Musk's political views are unrelated how well a Tesla car matches your transportation needs.

[spiderfarmer]

In the end it still boils down to how much you care about others and your environment.