I feel that perhaps you are being too cynical. There's such a thing as an insight snapping into place and recoding a lot of old information.
And there's such a thing as force building up for a long time against resistance, and then the resistance breaking; this is not sane, per se, but it's how I would describe my own sharp transition in 2003. I certainly don't think you could describe that as joining a social group.
Actually, I'd think there would be a lot of sources for sharp mental transitions. Just having to choose locally a preference between A and B will generate sharp transitions whenever A < B swaps to B > A and that means other things have to follow.
Yes, joining social groups isn't the only possible cause of sudden belief changes, but since the relevant info should have been coming out pretty gradually, it is still hard to see how a sudden large belief change could be that rational. I suppose one could more suddenly see an implication of evidence one had long held, but then the suddenness should be attributed to have realized that some point of view was possible at all. A sudden move to a point of view one had already recognized as possible would harder to describe as rational.
[I also mean this comment to reply to other comments besides Eliezer's but this system offers no easy way to express that.]
To break up the awkward silence at the start of a recent Overcoming Bias meetup, I asked everyone present to tell their rationalist origin story - a key event or fact that played a role in their first beginning to aspire to rationality. This worked surprisingly well (and I would recommend it for future meetups).
I think I've already told enough of my own origin story on Overcoming Bias: how I was digging in my parents' yard as a kid and found a tarnished silver amulet inscribed with Bayes's Theorem, and how I wore it to bed that night and dreamed of a woman in white, holding an ancient leather-bound book called Judgment Under Uncertainty: Heuristics and Biases (eds. D. Kahneman, P. Slovic, and A. Tversky, 1982)... but there's no need to go into that again.
So, seriously... how did you originally go down that road?
Added: For some odd reason, many of the commenters here seem to have had a single experience in common - namely, at some point, encountering Overcoming Bias... But I'm especially interested in what it takes to get the transition started - crossing the first divide. This would be very valuable knowledge if it can be generalized. If that did happen at OB, please try to specify what was the crucial "Aha!" insight (down to the specific post if possible).