Another possibility is someone had a packet sniffer running on your internet connection, your ISP's connection, WoW's connection, etc... and they picked up your and your wife's log in information. They can then keep sniffing and see when people log in. The fact that you haven't logged in in a long time tells them that you won't notice them hacking your account, thus they did.

Either way, I think someone managed to get some information a while ago and purposely waited for you, and anyone else whom they have their credentials, to become inactive so you wouldn't notice you were being "WoW robbed".