I had this happen to me years ago (Back in Orginal WoW)

I had a friend log in from the states and immedietly the account was locked out, I sat through the 4 hour phone queue and they told me their system was set to detect multiple IP connections to an account within a period of time) I assumed they had been doing this ever since, but seeing as I don't share account info anymore (I didn't know back then) I havent see it.