Probably as many different reasons as there are people who do it.

For a few it is confronting mortality they take up Pascal's wager.

I think though for most it is something else. Humans evolved to instinctively gather in groups.
Back on the African Savannah, this insured safety. Those instincts to be a part of a group are still present in modern day humans. In groups people generally feel more secure and "safer". They start to believe because it will gain them acceptance and a place into a group. If you consider people with Stockholm Syndrome, people will alter their thinking for the sake of survival. With religion, people alter their thinking to be a part of a group to feel "safe", as a result of animal instincts, which aren't so dormant as most people would like to believe. Those instincts tell the person to be a part of a group or community for survival.

The one positive attribute which religion provides is a sense of community and belonging.. I think it is ironic that the explanation for why people join religions can be explained via evolution.

Drugs can be an influence, Getting advanced monetarily or entering into a relationship can be the reason. Try this one out. How about being a Republican all of your life and leaving for good. I don't know of many people who never had a religion. You had a Holiday season at some time, went to a wedding, baptism or funeral at some time. I had racial indifferences in my lifetime as well as political and religious ones also. Most people are not aware of the connection that exists into becoming yourself if that is what you are in search of.

So it is fear and that would allow them fit right in with organized religion just another hypocrite.

It would be an interesting project interview those in this category and find out if there is or are reason(s) common them.

Can’t say I know of any who call themselves Atheist for whom this is the case. Most people start out with a religion...they may stop going to church and be rather apathetic about it, but they don’t really lose it altogether. When they think they may not have too long left before they die, they then return to the religion of their younger days as it never really went away, it just lay dormant. I don’t believe that type of person was ever a critical thinker, who deliberately and knowingly, rejected the notion that god exists. I feel it would be counter to any logical thought, but acknowledge that some may want to hedge their bets at the eleventh hour!

Edging their bets ... Notice how it's mostly old people in churches they have lived their lives mite aswell take out an insurance plan