Because of price drops in Tamil Nadu:
Indian growers dumping their tomatoes in lakes
Dheiva Sigamani, State president of the Tamil Farmers Union: “Farmers in Rayakottai and Valappadi, which are the big tomato markets are dumping tomatoes into nearby lakes as they are unable to recover their harvesting costs.”
Thenewsminute.com gives the reason for the drastic drop in prices: too many farmers opted to grow tomatoes -a short season crop- due to limited water for irrigation. This resulted in a dip in prices due to excess supply.
Dheiva also stated that the farmers were not agitated or protesting, but were simply disposing of the fruit as otherwise it would start to rot.