how very strange...

Security protocols

If you were logged in to Google+, that may be it. Idk the exact cause, but I usually don't have to re-log into (some of) the sites I log into when I come back from restarts or shut downs. I just think Google+ may be relevant to it. O:

Yeah, same here man. That also happened in my Facebook and Google+. They do this because somebody might get in into your account.

Well I'll have to agree with Davene Grovyle. I have the same reason.