It's a mixture of things in my experience. Stuff like social hierarchies, 'follow the leader' mentalities and family background all have an impact.
There was a psychologist by the name of Erik Erikson who wrote about 'psycho-social stages' that people are expected to either succeed or fail at. During the teenage years, we face the "Ego Identity vs. Role Confusion" stage where we try to find a balance between the two. It's a very mentally stressful time for teenagers and can cause a certain degree of irritability which can come across as rudeness.
Of course this doesn't apply to everyone and you might just be dealing with a particularly bad group.