Too much cargo, too little ballast water
Of the 476 people on board the Sewol, 304 did not survive the shipping disaster, including 250 students. The captain and various crew members, as well as the owner, director and four staff members of the Chonghaejin shipping company received heavy prison sentences. On the one hand for leaving the ferry with passengers on board, on the other hand for violating the loading restrictions. Instead of the maximum 987 tons, the Sewol was probably carrying 3600 tons of cargo. On the other hand, the ship most likely had too little ballast water on board.
The ferry capsizes after a sharp 180° turn was made. At a heel of 18° the cars on board started to move. The cars were lightly lashed according to the so called weather dependent lashing. The heel increased to 45° after which the ship capsized and shortly after sank.
The Dutch MARIN Institute built a 6 meter long free-floating model equipped with working screws, rudders and stabilizing fins. See the Downloads mentioned in the link with several papers.