What can cause all my accounts (facebook, SO, gmail, ...) to be disconnected all at once?
A few possibilities I can think of:


Cleared Cookies

Cookies are the basis of how a site remembers your session details. If you clear the cookies the session details are forgotten and you won't be logged in automatically when you next visit the sites.
Using a different browswer

Probably not the problem, but if you switch from chrome to safari or similar the session information isn't transferred.
The cookies expired at the same time

Sessions usually have an expiration time. It is possible they have all expired since you last visited those sites.
Someone signed you out

Hopefully this isn't the case. Some services allow you to sign out other logged-in devices. If someone did have access to your account it may be possible for them to sign you out of the other devices.