Pop crazy: What it is and why am I encountering it now?

(tags rubik's cube, practice)

Last sunday morning I enjoyed the sunshine at the local park, and I brought my Rubik's Cube (a speedcube). I spent probably about an hour grinding away since the sunshine was so pleasant for a mild October day. I kinda felt fast, because it was the middle of the day, I was sitting still and upright, and there was natural light all around me. However, sometimes when I scrambled my cube, an edge piece would pop out. This is the right definition of "pop". I confused this with unnatural sound of faces snapping together.

Since it wasn't during a solve, I thought it was just my rough and coarse turning habits. I think its just that my hands are getting stronger but not elegant. In any case it's not happening during my commute cubing time. I fear losing a piece on the bus and the embarassment of searching for it on all fours.