Verified answer

The main cause was the Partition of India into Hindu and Muslim states at the insistence of the muslim leader, Jinnah. Tens of millions of people criss crossed new and confusing frontiers in all directions and even though the British did their best to ensure order old enmities and bigotry took advantage of the general chaos and massacres sadly happened on quite a large scale.

Partitioning of the country into two dominions. Western (uninitiated) people look at it as though there were two peoples, which is the essence of 'two nation theory'. The name 'Hindustan' for India, became very handy for such theorists.

The populations were fairly mixed with varying proportions of Hindus and Muslims (and Sikhs in the case of Punjab). Cyril Radcliffe was given the task to draw partititon lines on the map based on majority of Hindus (+Sikhs) or Muslims, with red pencil in hand and a map in front, to decide the fate of almost a hundred million people. He was not given sufficient time. Most times the margin was close. There were no such borders anytime before and this is going to be a 'first'. The areas in question were Bengal (67 million people) and Punjab (32 million) provinces, as the fate of Sindh & NWFP (North-West Frontier Province) was decided to be with Pakistan as Muslim majority areas. The movement for Pakistan with 'direct action' (a euphemism for violence) unleashed a kind of religious (not ethnic) cleansing on unsuspecting populations who assumed that they'd live in their places for all time, having lived like that in the past. Large number of Muslims were in 'United Provinces' (UP) though they were in minority and they had to go to new and unfamiliar places with different language and customs, 'if' they opt for Pakistan. Many remained back. India made a conscious decision (contrary to popular western knee-jerk reaction as partititon appeals as neat & clean) of declaring the country for all religions and all disparate denominations. It implied that government didn't force any Muslim out. Nevertheless riots broke out leading to killings.

When the two - India and Pakistan, became independent on August 14/15, 1947 ordinary people finding themselves in the wrong country (though lived at that place for generations) started the exodus across the newly created borders with both streams going in opposite directions on the same highway. At times the columns of humanity heading in opposite directions used to attack each others (strangers to each other, but driven only by sectarian passion) leaving many dead. This continued for many months after.