The question is a bit strange. Most of the time, belief is not a question of choice. Belief is fairly subconscious, and somewhat invulnerable to the will. One can pretend to believe something for long enough for it to change, but it is not a simple political decision.

Many atheists did not "choose" to be one. They admitted they were atheist. Assuming they weren't life-long atheists, they just stopped believing because they were no longer convinced by the religious claims they were fed for most their lives.

It is like spending your childhood thinking that the Teenage Mutant Ninja Turtles were the most awesome thing ever, and over time, you slowly grow out of it until one day you realize you think it is actually corny and dumb.

Many atheists become so, because it is important to them to be able to verify that what they believed to be true, is actually true. They care that they are accurate. Thus, all beliefs and presumptions are called into question.