This happens to me frequently, I dont think it has as much to do with playing on different IPs as it does how you log in and off. I play on laptop and tower, I travel a lot so I like to play on the road. I rarely am forced to change my password unless my game crashes (whole different discussion, when will they fix the constant crashes jeez). Sometimes it also happens when I forget to log off at home (tower) and log on the next day with my laptop. So yeah, I think if you d/c, game crash, or log on with different IP while logged on (or closed game last session with d/c, or crash) it will prompt you to change password.

Anyhow not to bash the game, because I'm very hopeful things will continue to get better, but let's be perfectly honest... why would anyone 'steal' something that's worthless? The state the game is in doesn't give others a great deal of incentive to put the time and effort into hacking your account.