Why do some people in grocery stores try to ram you when they are in a hurry?

The grocery store itself is a institution of frustration and aggression. with raising food prices and fixed incomes I can see why people can be irate. Not to mention the long lines and sometimes heat. Other than that, people are just plain hungry and what to hurry up and get out of their so they can go home and eat...especially if they picked something up from the Wal-mart Deli.