When underground, zombies will teleport to the surface

I was testing out how zombies "socialize" by spawning around 30-50 of them throughout a massive cave. They did what i expected, until I saw the zombies popping up randomly on the surface. I believe they were doing this due to an error in the coding, but I'm not sure. I have done tests on this and it keeps happening. The zombies keep appearing on the surface above the cave.