The Powers. Higher Powers. The guys we work for, Kendra. Way past the Watches Council, the pull the strings around here. And there's supposed to be a war going on, between good, and evil. Evil being the Senior Partners. The Powers probably though you'd be good for the war and brought you back.
No. There was a battle against the First inside the Hellmouth. There was this amulet thing that destroyed all of the Turok-Han and collapsed Sunnydale. It as already evacuated though, before hand.