Tata steel was the biggest loser in the sensex pack falling up to 10 per cent after future prices for steel making raw materials dropped as China, the world's top steelmaker, indicated it could expand strict air pollution controls to more cities.

from Times of India https://timesofindia.indiatimes.com/business/markets/sensex/explained-why-tata-steel-plunged-10-today/articleshow/86369087.cms