Its not that biharis are uncivilised. Its only that it has entered the perception of people that biharis are like that only, which is not at all true. You can see biharis at some very top and respectful positions in the country.
For instance our first president Dr. Rajendra Prasad was also from Bihar. Who could say that he was a uncivilised man. This perception is based on behaviour of some of people from bihar who want to show themselves as unruly and that they don't care about rules(which might have different reasons for it, for example to gain popularity).
But we can't blame all people from bihar by seeing some of them.