Could anyone explain to me why some groups of Christians call their leaders "Father"?
It is out of respect for the type of relationship they are supposed to have with the people that they lead.
There are several places in Paul's letters where he says he has become the Father of those who he leads spiritually (1 Cor 4:15, Gal 4:19, Phlm 10 ). But no where where it is exhorted in general to call the presbyters or elders by the title "Father".
Some people say Jesus says not to call anyone "Father", including Church leaders. But the reason for calling a church leader "Father" is spiritual fatherhood. So all the things you can think of that your father is supposed to do for your upbringing and sustaining your body a church leader is supposed to do for the upbringing of your spirit and sustaining your soul in friendship with Jesus.