This happened to me tonight during a family dinner I was hosting. I was using my Apple TV to stream Apple Music via AirPlay and then EVERYTHING (nine total devices) was booted out of iCloud. Wife’s and kids’ devices were fine. I thought my account was being attacked in real time. Given the recent password-reset hacks, I thought I was a victim.
I solved it by resetting my Apple ID password, using 2FA direct from Apple on one of my other devices.
*HOWEVER*, I was also receiving SMS codes to reset my account during this time, which I absolutely wasn‘t going to risk using and seemed odd to me. Maybe they were legit, but I wasn’t going to follow that road with anything less than 100% certainty.
ETA: My account is an old mac.com account, before MobileMe. Wife and kids are iCloud.com accounts.
NB, anyone with a physical SIM who uses their phone to receive important SMS codes related to authentication, please ensure that you have SIM lock activated. Settings | Mobile Service | SIM PIN >
Otherwise, if you are unlucky enough to have your phone stolen, someone can put your SIM in their own phone and get up to all kinds of tricks, including obtaining reset codes sent via SMS.