We had one 10x developer in our team (it was a bigger project with about 70 developers). His output was 10x the average mainly because he had very (both) deep and broad domain knowledge, i.e. knowing largest part of our own code base. That will allows him to produce much more, since he doesn't need to investigate how something was done or how to integrate the new feature into the system. He was also quite hardworking, which also helps.

Of course, if he would be put into a completely new project, he probably wouldn't be 10x, at least not the first few years there.