When I becomes We

How about mob psychology for a topic. When you are in a group and a few people in the group turn violent or start doing something, how many times have you noticed that the rest of them just follow suit. It could just be a small provocation. And most of the people there might not actually have wanted to do this.
There are innumerable examples too. Take for instance the recent monkey chants in Mumbai. A couple of people start it and the entire section of the crowd joined in.
There are people who say that the recent molestation on New Year's eve in Mumbai was also something like this. The crowd was huge and when a couple of people did this, the rest of them joined in to have a piece of the cake.
I am sure most of you might have been witness to mob psychology one time or the other. This need not be physical violence of any sort. But just for instance take a college festival. With some one performing on stage. A few guys in the crowd begin boos and cat calls, have you not seen a majority of the crowd join in? And this majority may not actually endorse what they themselves end up doing.
On the positive side, when people start chanting "Once more Once more" at the end of a performance, you too are dragged into that, even if u'd actually prefer some change :D:D.But what is actually mob-psychology. Why do people actually behave in this manner? It might be upto a psychologist to answer this.
Wiki says "Individuals tend to behave in a different manner as part of a group in contrast to acting independently. Members of a group are prone to acting in ways that they would deem immoral or unjust if in control of their behavior".
So is this really something that a person can not control no matter what? Can people actually justify themselves later on, blaming "Mob-Psychology" for it? How would you react if you were at the recieving end of this?

What made me write this: Yesterday a BMTC bus ran-over a 3 year old kid near JC Nagar in Bangalore. What happened next was a huge mob of people stopping all busses passing that way and virtually destroying them. They even damaged a lot of public property, vandalizing the road divider and other traffic property.So is this act justifiable?