Skeezyks put it very well. Built up pressure from stress/anxiety/etc, it's like a shaken soda bottle. And it goes off at all the wrong moments.
For me, personally, I have found that humor is a good combatant to anger. Especially when I feel that pressure build up.
My therapist had me read a book called 'The Cow in the Parking Lot'. It was an interesting perspective.
Worth a look if you'd like.