1000s to 10s of 1000s of sikhs were slaughtered by hindu mobs in 1984/5...The most bloody halloween episode.....The Sikhs were once considered allies of hindus in the latters' conflict against muslims .....and Sikhism is said to be a sect of hinduism according to india constitution... Basically they turned an ethnic ally into a new enemy. 

link: Remembering 1984 - Sikh Coalition