A: Atheists by definition are people who don't hold a belief in any god, so in their minds there is no "God" with whom to be angry. This is a bit like asking a Christian why they're so angry with Allah.
You will, however, often encounter a hostility to religion among many atheists, which is a subtly but significantly different issue than being angry with God. You can be angry with what, for instance, Muslim terrorists do even while not believing in the god they profess to worship.
Without making a judgment as to their merits, here are the reasons I most commonly see given by those atheists who seem to be hostile towards religion:
- Harmful Actions: Religion is seen as giving adherents license to commit unlawful, immoral, or otherwise harmful acts against other human beings. Regardless of the existence or non-existence of the god worshiped in that religion, the acts themselves are to be deplored on their own merits. Religion is regarded as significantly more likely to encourage such behavior than other belief systems, and as such should be avoided.
- Harmful Thought Processes: Religion is seen as promoting non-rational, non-scientific, non-logical modes of thought that lead to less than desirable social consequences. One example often given is the effort to replace the scientifically solid theory of evolution with Biblical "creation science" or "intelligent design" in public classrooms. This lessens the ability of our students to think critically about the world around them, in turn weakening our country's ability to compete in the scientific world.
- Truth Matters: Strong atheists believe there is no god, and to allow significant social institutions to go around promoting an untruth is distasteful. In general it's better to be truthful than otherwise, and this is just one of the more prominent examples of culture promoting untruths.
Those are among the more logical or rational reasons you'll find atheists promoting for why they dislike religion. There are many, many others, some much less rational and more emotional. Sometimes people who are raised in a very religious home will rebel against those teachings later in life, perhaps as a way of distancing themselves from their parents and perhaps out of genuine revulsion at the practices themselves. Some atheists undoubtedly are angry with what their family or community did to them in the name of religion, and thus reject all religion as a consequence.
Again, the Universal Caveat applies; you're going to get almost as many answers to this question as there are atheists out there. Including, of course, the sizable group of atheists who don't give a fig about religion one way or the other, any more than non-stamp-collectors care about the goings-on among philatelic groups.