I have a used strop, it came to me with 2-3 little nicks in it. Today I took a close look, and it seems that I've added a few. These aren't sections of missing leather, but rather just little slices into the edge, mostly at the bottom of the strop. My question is how in the world am I nicking my strop? It just doesn't make sense to me. Watching my motion, as near as I can tell the blade isn't moving away from the spine at any point.

How do nicks in strops happen? What am I doing wrong?