Why did the murder of a preacher in a Sikh temple in Vienna spark riots in the faraway Indian state of Punjab, in which thousands took to the streets to torch cars, trains and battle security forces?

The root cause may lie in India's caste system that Sikhism officially rejects, but that still grips swathes of India's billion-plus people, including in Sikh-dominated Punjab state in northwestern India.

"Via Vienna, Sikh caste war returns, sets Punjab aflame" ran the headline of the Hindustan Times.

The preacher, Guru Sant Rama Nand, 57, was killed in a gurdwara in the Austrian capital in an attack by six men armed with knives and a gun.

He was from the Dera Sach Khand, a religious sect separate from mainstream Sikhism that has a large support base of Indian Dalits, or "untouchables", and other lower castes.