I'm not sure why it continues, either. It may be that people are trying to identify with Christ and have decided this is one way to do it. I think it's sad, since Christ did this to atone for our sins so we would not have to suffer to atone for our own sins. Some people may do it to try to show their repentance, but God certainly doesn't encourage this practice. The Paul, in Galatians 2:20, says, "I have been crucified with Christ (past); it is no longer I who live, but Christ who lives in me; and the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me.
I don't think this is Satanic, but I think it is unnecessary and the only spiritual gain I could possible think of is that someone might have a small idea of the debt they owe Christ. Even were someone to be crucified to death today, he would still not begin to fathom the pain Christ went through for us in bearing the sins of the whole world, facing the wrath of God for us, and being cut off from his Father during that time. So I see this as a kind of futile effort. I guess I wonder more about the people who drive the nails in.