Atheists are a diverse bunch. We really only agree on one thing, which is agreement on a non-thing. In every other way we won't necessarily agree. Likewise, we come to our non-position from many directions. Social pressure to believe in the prevailing religion is so strong that for most of us deconversion is a gradual and personal process.
Family ties: some of us had the good fortune to grow up in freethinking homes. As religion is ubiquitous in society (currently), this is the best way, because children will have the option to convert if they want but won't feel they'll go to hell if they do.
Reading the Bible: The more you know the Bible and the more you know about it, the more you see its problems. This is why churches have "Bible Study" and selected brief readings during services: if you read it without "guidance" you would reject it. This is also why so many pastors have become atheists.
Meeting an atheist: If you've never known an open atheist, it may not even seem like a viable option for you. Even worse, churches teach falsehoods about atheists (aside from the Hell-threat). At the time I fully embraced non-embracing of religion, I had only met two "out" atheists in my entire life, and I was about 28 at the time! I'd known a few wishy-washy deists and some befuddled agnostics, but only two people had told me they just didn't believe in any god. Meeting one atheist could be all it takes to self-identify when you realize your disbelief is not so unique.
Science (?): I wonder how many scientists become atheists after starting their careers. Richard Dawkins deconverted long before he chose his career path. I'm not familiar with enough others to know if this is common. It would certainly be difficult to reconcile astronomy or evolution with the stories of the Bible, but there are many scientists who are believers. The power of rationalization keeps many believers in the pews.
Here are some ways we typically don't become atheists:
Anger toward God: If a Christian's prayers are not answered, or something unexpected happens, they may feel some anger toward God, but self-blame is so hard-wired into the theology that the typical Christian turns that anger towards themselves. They will decide they didn't pray for the right things, or that they are too ignorant to know what's best. They soothe themselves with trite consolations: "In the end, it's God's will." "He needed another angel in Heaven." "One door closes, and then another opens." If you don't believe in any god, it's impossible to become angry with one.
Bitterness: We aren't bitter about life, though we are sometimes bitter and angry about being fooled by religion. People who have been in cults may feel they were robbed of life experiences during that time, and who could blame them? But that won't turn someone against God. People who are angry with one religion or bitter about the way they were treated by their church will find another religion, denomination, or parish if they feel put-out but still believe.
Disillusionment with church leaders: The Catholic church's pedophilia problem is just one of many problems endemic to a group of people with virtually unlimited power over another group of people. Mega-church leaders and televangelists have been caught with their pants down in sex scandals, too. I have known a lot of Christians who are forgiving toward these "fallen" leaders, or at least rationalize their behavior as being individual weaknesses rather than an indictment of belief. One Catholic friend converted to the Greek Orthodox church after the pedophilia charges came out. Hypocrisy is something the believer can live with. The only thing they can't live without is belief in a deity.