I'm not sure that your rephrasing accurately captures what I was trying to get at. In particular, strictly speaking (*) doesn't require that one be a part of a group , although being part of a group often plays a role in enabling (*).

Also, I'm not only interested in possible irrational causes for LW/SIAI members' belief that future AI is one of the most important things to worry about, but also possible irrational causes for each of:

(1) SIAI members' belief that donating to SIAI in particular is the most leveraged way to reduce existential risks? Note that it's possible to devote ones' live to a project without believing that it's the best project for additional funding - see Givewell's blog posts on Room For More Funding:

For reference, PeerInfinity says

A couple of times I asked SIAI about the idea of splitting my donations with some other group, and of course they said that donating all of the money to them would still be the most leveraged way for me to reduce existential risks.

(2) The belief that refining the art of human rationality is very important.

On (2), I basically agree with Yvain's post Extreme Rationality: It's Not That Great.

My own take is that the Less Wrong community has been very enriching in some of its members lives on account of allowing them the opportunity to connect with people similar to themselves, and that their very positive feelings connected with their Less Wrong experience have led some of them to overrate the overall importance of Less Wrong's stated mission. I can write more about this if there's interest.