Why did the crew of the zong ship throw some captive africans overboard?

The Zong massacre was the mass murder of 133 African slaves by the crew of the slave ship Zong in the days following 29 November 1781. The Gregson slave-trading syndicate, based in Liverpool, owned the ship and sailed her in the Atlantic slave trade. As was common business practice, they had taken out insurance on the lives of the slaves as cargo.