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

Keep in mind that sometimes we confuse passion for anger. The people, on any side of the social/political spectrum, that one hears is likely to be ones that are impassioned about a topic.

It is only a question of perception, mixed with some confirmation bias.

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

It is important in any discussion not to dismiss (as in an Ad hoc argument) what one is saying because they seem to be "angry" about it.