What causes itchy white bumps behind ear lobes? I can pop them and the white stuff comes out.

Sebaceous cysts. Many people have these cysts, which develop in the earlobes and behind the ears. They are oily, containing sebum, and sometimes can get infected. If you're able to express and drain them on your own, I would not be too concerned. If you develop a large one or it becomes infected, then sometimes surgical removal is necessary.