What is the root cause of ennore (Chennai) oil spill?

On January 28, 2017,two cargo ships collided off the Ennore coast in Chennai causing oil to spill into the sea. The spill has been polluting Chennai’s beaches too.

Due to wave action and the southern current, the spill spread some 34 km till Vettuvankeni in the south, making patterns in the sand. At least a thousand volunteers have been removing the sludge