The kind of person I can see maintaining Truecrypt for a decade I can also see making this decision. They're upset and are completely done with the project. They take down all the old versions, knowing that the licensing means they're largely useless for purposes of forking (unless you want to violate the licensing, which causes questions to the validity of the fork). Despite their frustration with the project, they still deeply care about crypto and keeping their users secure, so they publish some brief recommendations on alternatives, and release a read-only version of their software to support it.

There are any number of possible vulnerabilities that could exist. Its definitely a plausible possibility. I could fairly easily believe that they were contacted by a researcher who was about to publish a major AES flaw, or one of the other algorithms in use.

There's a number of relatively plausible theories. I wouldn't be surprised if we don't find out for 20 years what the actual reason for this was, when the developer is on their deathbed.