Another reason for resentment is folks from UP/Bihar dont learn the culture and language of the state they settle in. They expect everyone to speak Hindi with them. True, Bollywood is popular all over the country , this doesnt mean everyone in India will accept Hindi at the expense of mother tongue.
One other reason is that because of Hindi, people from Bihar and UP have an advantage in Central jobs like Railways, Central Government etc. These employe a large number of people and are dominated by UP/Bihar folks.