Account locked for change in access pattern?

Hello. I wondering if I can get some more info on this. My wife and I both have an account each and play on different computers in Taiwan. We play on the Oceanic servers. My computer is better and sometimes when I'm at work, she'll log on my computer with her account and we'll play together on separate with our guild at night. We've never had a problem and have always been very cautious with our accounts and computer safety. We recently switched internet providers to play the game more smoothly and I guess the ISP on my computer has changed.
She hasn't finished the current patch because her hard drive ran out of space. Yesterday, she tried to log on my computer and the message "account locked for change in access pattern". We went through the account recovery, reset her password and email password and got her playing again.
I got kind of spooked and ran very anti-virus, malware, spybot and Avira program and everything came up clean. Yes, I did it with the WoW program open.
Today, it happened again. She reset everything again. Then, I logged on and the same message popped up.
I ran another virus test. Everything was clean. I reset my password and email password. No sweat.
SO, my question is this happening because we've changed our internet provider with a different ISP address. Or, is it because my wife's account hasn't been patched in her computer and when she tried to log on mine, it showed a red flag? We just a bit concerned and would like some proper feedback.
Has WoW become so tight-noosed in preparing for the next expansion?