Having atheist views and then maybe coming into or paying interest to sikhism seems to be a little more common.
I myself at one time had some very strong atheist views. Paying more interest to sikhism seemed very attractive as the truth was away from all rituals and other beliefs.
I have received positive responses to sikhism from a few atheists that I have explained the religion to.
There are many reasons why people choose to be atheist.
Most popular is probably to get away from rituals, celebrations etc ...that these people see as not being to truthful.
I've often heard examples of someone seeing a person make such a fuss about being at church every Sunday or something and then seeing them the next day trying to sell a dodgy car or something to a vunerable person. This kind of activity puts off a lot of people and leads them to atheism.
Maybe the fact the true sikhism focuses on truthful living and a clean way of life is what makes it attractive for some atheists.