Account locked due to suspicious activity?

So when my fiance and I went to log on tonight, we received a message telling us the account has been locked and that we needed to change our password, so we did exacly that. Once the password was changed a few moments later we were told the account is locked again, and the password needs to be changed again, so we changed it again...

But were still kind of both confused as to why we had this happen in the first place, I mean the email that was sent from blizzard tells us to secure our email, secure our password ect ect ect Im sure you are fully aware whats in your emails.... But as far as we knew theres nothing wrong with our emails we have no viruses we only share our account with each other being as though we are a couple so I can't find any good reason for us to be getting locked.

What is it exacly that makes one account suspicious and another not? Because as far as i know farming rep for mounts and raiding isnt exacly against the rules.....? Any information could be helpfull being as we play the game like everyone else does, except were getting locked and they are not.