It's the most common assumption made in regard to atheists - we are angry. We are mad, curmudgeonly angry people! Is this true? Well, in some cases of course, because an angry disposition is a common personality trait. The fact is, however, that the trait transcends belief or non-belief. Some people are just angry, and some of those some people are indeed atheists.
The reason why the "angry atheist" stereotype bothers me so much is because of personal experience with being accused of being one of these kinds of people. I have been told that I am an angry atheist at least two times that I can remember, and I was angry when the person accused me of being angry, I fully admit that. I was angry on both occasions because I had already suffered through a long conversation where a theist had no problems insulting my intelligence and my morality while simultaneously presenting unproven information as facts and distorting the meaning of language in order to prove their points.
This is an old christian debate trick - trip them up in so much BS that they (hopefully) become too tired and distracted to be able to continue to argue coherently, then claim to have won the debate. Make sure to pepper responses with overwhelming and unapologetic condescension and disrespectful assumptions on the atheist's personality, personal life, and general morality. Make sure you tell them you pity them. Then, when they lose their cool and return the admittedly inappropriate personal attacks, write them off as 'just another angry atheist.'
The trick here is to have the willpower to walk away from a debate if you sense that the conversation is less an exchange of ideas and more of a person with an opposing viewpoint looking for validation through any means - regardless of the validity of their reasoning and information. However, this can be more and more difficult to do in correlation to how ridiculous the claims being made are. It's a fine line between refusing to be silent and speaking up for the atheist point of view and speaking yourself right into a malicious trap meant to spur a reaction from your ego by any means necessary in order to fallaciously marginalize you as 'just another angry atheist.'