Oh, by no means I am not saying Atheists are angry all the time. I have many jolly atheist friends. I think it just comes out mostly when discussing religion. I don't get mad when others don't share my views, but it seems like atheists get angry at believers of any type. They like to stereotype a lot. It could be frustration or bewilderment, but it comes off as anger. Maybe I am stereotyping as well. If so, shame on me.
I am not alone in this thinking. Maybe it is just in the believer community, but that is the perception. It's not just me.
https://www.google.com/#q=why+are+atheists+so+angry