What makes a boil pop?

Pressure & erosion: Likely a combination of processes. An infection or inflammed area exudes chemical messengers that draw white blood cells in to do battle, so fluid builds up. Also, one way the body trys to heal itself is by drainage of what it can, so the tissues thin and rupture to let the pus/fluid/infection out.

Abscess Treatment: An abscess is a tender mass generally surrounded by a colored area from pink to deep red. Abscesses are often easy to feel by touching. The middle of an abscess is full of pus and debris. http://www.webmd.com/a-to-z-guides/abscess#1