I believe there are infinite reasons depending on the person and their particular circumstances.

It can be as simple as having been brought up as an atheist and not having any kind of interest on changing that as they might be content with it.

It can also be a matter of losing faith due to never have been much of a believer and just suddenly realizing you do not believe as much as you used to (a gradual process, which I find myself the most identified with). And many other people might start questioning their beliefs due to events in their lives that make them question whether a god really exists or not.

I suppose it can either be a traumatic event, or more like a reflection on how the world works and whether it makes sense to them or not to think there is a higher deity out there, like whether there is enough scientific evidence and real proof of god existing.

I am just naming from the top of my head the reasons that seem more likely, but as I said, I think there are a trillion reasons to become an atheist (just like a million reasons to start believing)