If I recall correctly, his Dorian Nakamoto message was around the time that there were some threats to satoshi's safety. As it says on the p2p foundation thread:
Dear Satoshi. Your dox, passwords and IP addresses are being sold on the darknet. Apparently you didn't configure Tor properly and your IP leaked when you used your email account sometime in 2010. You are not safe. You need to get out of where you are as soon as possible before these people harm you. Thank you for inventing Bitcoin.
Whether or not this is true I don't know, but it's certainly possible that satoshi's p2p foundation account was compromised to make that Dorian Nakamoto comment. Someone even
got access to his email, as theymos says.
If you ask me, satoshi vanished when he vanished from this forum with his last post on December 13th, 2010. He could just want to leave BTC without one definitive leader (in the name of decentralisation); he could be dead; he could have been afraid of authorities.