My dog scared the crap outta me.?

I own a boxer/pit mix. He's only 7 months old and sleeps with me. Rite infront of the window. Today at 6:40am he jumped straight up, ran to the window and started barking like crazzii. I've heard him bark b4 but nevr like this. He wuz howling and everything and I kept pushing him away but he held his ground. Anywayz when I looked at him I noticed tht the hairs ontop of his tail wuz standing up, and the hairs on his neck were to. Wht does tht mean.? And wht wuz he so scared of if there wuz nuthin outside.?