Don’t panic, there has not been some mass hack or anything like that, ArenaNet are wanting to make sure all Guild Wars 2 players have secure passwords.
Last September ArenaNet added a blacklist system where common passwords or passwords hackers had used previously were no longer allowed when setting up an account. Now the system is in place, ArenaNet want to make sure all Guild Wars 2 players are not using any of the passwords in their list so accounts stay secure. Note this only applies to anyone who has not updated a password that was set prior to 12 September 2012.
On 7 February a password change will be mandatory but it’s probably a good idea any passwords are updated now via player’s account management screens. Better safe than sorry.
Source: Guild Wars: IncGamers