It´s charity time yet again and not only this, it´s bargain time for gamers too. The Humble Bundle is packed again, this time full with SEGA games and you can grab them and decide what you want to pay. The amount will go to the charity. But as often when gamers go crazy to get their Humble Bundle, Steam will suffer from it and goes down for a while.
That´s at least my assumption as I experienced it several times when there was a bargain Humble Bundle. People want to activate all their activation keys they got from the bundle, and that at the same time! It might be the reason why Steam is down again. With a little bit patience, everything will be right soon I guess! You can by the way check the steam server status to find if the Steam servers are really down.