It looks like it's similar to the issues steam had a while back (https://www.destructoid.com/-updates-psa-steam-is-giving-peo...)

While pure speculation until Chase makes an official comment: it's probably a caching issues somewhere backend on their system. Sessions are getting mixed up which means, if it really is a caching issue, not logging in until the issues are resolved should prevent your account from being logged in by a random 3rd party