It seems these groups exist, in large part, as an effect of their beliefs about the biggest risks. You're not afraid of global warming and pollution because you are an environmentalist, rather you are an evironmentalist because of your fear of global warning and pollution.
That said, I'm not sure what your point is. I'm sure there are many in each group who haven't done the math themselves and are just following like sheep. But it is the same regardless of what we are talking about...it certainly isn't specific to evalutating threats to humanity. Just groupthink and half a dozen other biases at play.
The other thing is that groups may not be focusing on the largest existential threat at any given time. Instead they might be spending time on a particular issue that has come to the forefront.
Conservative Christianity, for instance, is dealing with homosexuality right now. But that is really just a pawn in a much larger eschatological endgame. Homosexuality isn't really that big a threat to Christians. Hell is a bigger threat.
You're not afraid of global warming and pollution because you are an environmentalist, rather you are an evironmentalist because of your fear of global warning and pollution.
I actually think the former is more true than the latter. You first become an environmentalist (through e.g. social pressure and status-seeking) and then filter your information input to become fearful of global warming and pollution.
If I were to ask the question "What threat poses the greatest risk to society/humanity?" to several communities I would expect to get some answers that follow a predictable trend:
If I asked the question on an HBD blog I'd probably get one of the answers demographic disaster/dysgenics/immigration.
If I asked the question to a bunch of environmentalists they'd probably say global warming or pollution.
If I asked the question on a leftist blog I might get the answer: growing inequality/exploitation of workers.
If I asked the question to Catholic bishops they might say abortion/sexual immorality.
And if I were to ask the question on LessWrong (which is heavily populated by Computer scientists and programmers) many would respond with unfriendly AI.
One of these groups might be right, I don't know. However I would treat all of their claims with caution.
Edit: This may not be a bad from thing from an instrumental rationality perspective. If you think that the problem you're working on is really important then you're more likely to put a good effort into solving it.