I think most popular reason would be failure to get into good paying signature campaign, or a reduce of profitable bounty campaigns. Or "earning using forum" in general.
That's most likely true for the majority of members, and your example of cryptotalk was an appropriate one. As soon as people stop getting paid to post (or can't manage to get into a campaign or bounty), suddenly their motivation to be a member leaves them completely. Personally I'd keep posting here regardless. I didn't register on bitcointalk with the intention of ever joining a signature campaign and really didn't know much about them until I'd spent a few months on the forum. The only way I'd leave is if I found that I'd outgrown the forum or if something in my life prevented me from being able to post here (or if I got banned, of course).
Great example is Cryptotalk forum, that was once advertised here in signatures. Due to lack of bounty campaign (last bounty campaign on that forum ran more than half year ago) a number of users left that forum. After forum owners reduced the amount of post reward, great amount of user left it. Only spammers and those who have high ranked accounts and use forum for referral link promotion are left there.
There were never anything
but spammers and plagiarists on that site. Basically, cryptotalk.org took the bottom-feeders from this site and tried to create a workable forum around their membership--and it failed spectacularly. I haven't visited cryptotalk in a long time. Is it even still active anymore?