Ask HN: Logged out from multiple sites in more than once browser

Seems over night I have been logged out from multiple sites (Including HN, Google and GitHub) in at least the two browsers I use daily.

Still logged in on some other sites, e.g. twitter.

Am I missing something obvious and/or has anyone else seen anything like this before (i.e. happening in more than one browser and more than one site at the same time)?