It might not be obvious to everyone at first, but there is a connection between atheism and vegetarianism. Think of it this way. Many people believe in one or more gods because they grow up around people who believe, worship, and sometimes offer sacrifices to those gods. This is something that is inherited from the culture around them. Meat is exactly like a god. People eat one or more kinds of meat because certain animals in their culture are routinely killed, sold, and eaten. For example, I grew up in the USA where it is common for people to eat cows, chickens, turkeys, and pigs. Why eat these species and not kangaroos? Probably because there are not many kangaroos around here.
Some cultures eat dogs while others see dogs as pets and would never think of eating them. If you eat meat of any animal, can you explain the causes behind why you eat certain animals and not others? My question is what is restricting carnivorous humans from eating other humans? If you look at it, that makes perfect sense to a carnivore. When I was about twenty-five, I asked that question and quickly realized that I was not comfortable with the idea if eating humans, vegetarianism was the inevitable result since then. I could not give one reason why it was okay to kill and eat a turkey any more than it was okay to eat a human. Basically, it follows logically that to oppose the killing of humans, the minimum requirement is to oppose the killing of all the other animals as well.
How does this relate to atheism? The primary reason being that one who rejects animal killing has basically rejected any and all institutions which promote the practice. It means the rejection of all religions that promote the sacrifice of animals or the eating of them as part of a religious holiday. It doesn’t disprove the existence of gods, but it does explain why many atheists have become vegetarian and others vegan. If you leave a religion first(like I did), you may stop the carnivorous traditions from that religion. The reverse is also true, if you hate the killing of animals, you will automatically have a problem with holy books which promote the practice of killing or hurting animals. Once again, this does not prove that gods do not exist, but it means that you will reject the moral code that you were told was given by a divine lawgiver.
I would expect a god to prevent the torture and killing of all the animals who are conscious and feel pain. A god who does not do so could theoretically exist, but it doesn’t matter to an animal rights activist because that god is seen as immoral. So while caring about the suffering of sentient life does not make someone an atheist, it sure gives atheist vegetarians more reason to eliminate religious beliefs that lead to more killing. If you are vegetarian(eat no meat) or vegan(no animal products), your only theistic option is to believe in a god that is too evil, apathetic, or powerless to defend life. This was unsatisfactory to me and is why it matters not a bit to me if any gods exist because it has no relevance to this life.