My neighbor used to load and unload these ships. Back in the day, he would hire me for a day to off load Benzs here in Baltimore. I've actually driven over 200 Benzs albeit for less than 1/3 mile each.
Other than that, I am not an expert but I do know that these ships need to be loaded and unloaded in precise sequence to maintain stabilty. Some of these ships can transport 3000 cars. The offloading is very carefully controlled.
I would imagine however that it was something else. My hunch is that it happened out at sea. Maybe rough seas in combination with an inexperienced crew?
Sad though. I feel for those with cars on board. At least you have alcohol!