Atheists aren't any more angry than the rest of the population.

Keep in mind that sometimes we confuse passion for anger. The outspoken people - on any side of the social/political spectrum - are likely to be ones that are impassioned about a topic. It's only a question of perception, mixed with some confirmation bias.

Atheists perceive many Christians in the U.S. as continually pissed at anyone who isn't following their doctrine. Many Christians see atheists as "flaming", and attacking anything resembling tradition or "values".

It's important in any discussion not to dismiss what one is saying because they seem to be "angry" about it.

Although, if you're curious about the things that do anger atheists, the book "Why Are You Atheists So Angry? 99 Things That Piss Off the Godless" by Greta Christina, covers the topic fairly thoroughly.