Aggressive atheist

I just met my first truly aggressive atheist. I was having lunch in an Ashfield restaurant with Peter Gibson, one of the guys from church, and we were talking about the gospel and schools. While we were minding our own business, having a quiet chat over lunch, he came up, stood over our table and basically started hurling abuse at us. Words such as arrogant, stupid and deluded were strung together in sentences that came think and fast. Wow! I think if I had done a similar thing he would have called the cops.

In the end we a a brief conversation (where it felt that Peter was ten times more gracious than I was) before he had to return to his work. I had never really met a full blown agro atheist in the mould of Richard Dawkins but now I can say I have. What I want to know is what drives it? Any thoughts?

PS. It was really nice that he left me his card.