Racism, also religion and simple political and cultural differences.
There is considerable venom in some mouths over India/Pakistan.
Wiki wrote:The two countries have a complex and largely hostile relationship that is rooted in a multitude of historical and political events, most notably the partition of British India in August 1947. The India–Pakistan border is one of the most militarised international boundaries in the world.
In Britain this is not seen as much (though I have seen it, especially with middle-class Indians and working-class Pakistanis), but it exists, obviously more pronounced in the minds of extremists, of which Braverman must be counted as one.
There is a lengthy Wiki entry on India-Pakistan relations.