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.]