The servers are locked again because people insist on moving to them and staying there.
If you play on one of the most populated servers in the game, then you should know by now that it will get locked from time to time.
You even got free transfers recently if you wanted to move to some less overpopulated server.

The recent reorganizing of the data centers had nothing at all to do with certain servers being full, but was to reduce the load on the instance servers in preparation for the upcoming expansion.