It’s actually quite common for people to convert from one religion to another, from no religion to a religion, or from a religion to no religion. The reasons probably are as varied as the number of people but most all decisions are personal ones. For example, two people may want to get married and raise children but one spouse needs to convert to match their spouse’s religion so that they can raise their children in a household that shares the same religious beliefs.
Some people may stop believing in religion because they just can’t understand how a “God” can let so many things happen to so many good people. It just doesn’t make sense. Others may see a miracle or something wonderful and believe the only explanation for it is that “God” solved the problem so they start to believe when they never believed before.