Why did my lab defend me?

Okay, so I have a yellow lab. His name's Bullet; seriously the sweetest thing. Except for last night. As I was giving Bullet his nightly walk this guy came out of nowhere and punched me in my face (not a good feeling) and aimed to mug me I suppose. But Bullet was NOT a Bullet I've ever seen. He's usually the standard sweet, expressive, good with kids, happy-to-meet-you kinda dog, but last night he was LIVID. If I didn't hold him back, I don't know how bad that guy would have had it. I'm just afraid somebody might piss him off on the wrong day or something, but I'm glad he defended me. This isn't normal for labs is it?