I see this question asked by religious folk frequently. I do disagree with some of the harsher anti-religious sentiments, at least as far as tone is concerned, but I understand exactly where those attitudes come from. Christians (speaking from the USA) have a harder time seeing the hold their religion has on our society and the actual harm it can do. When they are faced with the topic of crimes and abuses committed by other Christians, I commonly hear that the perpetrators were not "true Christians."
What is a true Christian? If a person, even if they are a criminal or an abuser (or, heck, just a run-of-the-mill wacko), professes belief in Jesus as the son of God and the savior of humanity, and accepts the Bible, then are they not a Christian? The same people who are quick to point out that others are not true Christians are, more often than not, just as quick to point out the beliefs of other wrongdoers - Muslims and so-called "Satanists," for example. All people are capable of committing crimes, regardless of belief, but those who commit them in the name of their religion should not be ignored or brushed off as "not true believers." Just as it provides light and hope to some, there is a dark side to religion that people should acknowledge, too.
So back to the question of why atheists are so "toxic." When religious people lurk in atheist groups and spaces, it can certainly seem that way to them. But we speak so harshly because these areas are some of the only places that we CAN. Some among us have been hurt and abused by religion and religious people. Many see the religious attempting, at the expense of other people, to impose their standards of morality on others through law and social norms. Why are we told we need to "respect the beliefs of others" when ours are denigrated?
These areas are for us to freely vent our frustrations, as well as share our experiences with others of like mind. Atheists should not have to be polite about religion even in a space made for themselves. Certainly people should be able to freely believe what they like in their homes and in their hearts, but that doesn't mean religion should be free from criticism, and it doesn't mean religion is 100% good all the time. Nothing is.