The servers being locked or not seem to be varying and/or rotating. Just a wild guess with no evidence to support it but I think maybe they are temp-locking some servers just to try to steer new characters to some that may not be getting as much traffic. I was trying to make a character on Ultros earlier and it was locked, I went and got a drink and threw a load of laundry in the washer then refreshed the server list and Ultros was back up and available. I just imagine some guy sitting at SE HQ saying "lock Ultros, Excalibur, and these other servers for 10 minutes so we can get some people to join Goblin and Leviathan".
This one time at FFXIV camp I tried to get a 3 person party going, but just ended up as a Disciple of the Hand.