Anger is one of the emotions that many of us have a hard time owning. If you've been hurt in the past but never allowed yourself to go through the process of feeling the anger, the anger's still there. Because there is no outlet for the anger, you become a hostage to it.
"Whatever emotion you don't own will own you"
As hard as you deny you have unresolved anger, it's always right below the surface. Things such as being caught in traffic or a long line at the store, are some of the triggers that set it off.