Why did the British partition of India cause a refugee crisis?

There had always been tension between the Hindus and Muslims in India. When it was made known that the British rule would end, it was decided that the majority of the Hindus would stay in India while the Muslims would transfer to the newly created Pakistan. More than ten million people had to leave their homes and move to the other state. Having so many refugees created riots between the Hindus and the Muslims and more than a million people died.