For some reason, the hull of the submersible had failed and the high pressure water rushed in and ripped it apart. The crew were very brave to go there, the water pressure is so powerful when you're several killometers under the ocean. I doubt they will ever find the bodies, they probably floated half way to the surface and got eaten up by sea creatures, the force of the deep ocean would have instantly torn them to pieces. I would put their cause of death as the Titanic. They went down there to look at the Titanic that had sunk on 14 April 1912. 111 years later it had claimed 5 more lives.
Five brave explorers died in the deep ocean because of the sunken Titanic, the largest ship that existed in 1912. If the Titanic had not sunk, they would not have gone down to the sea floor to look at it and their submarine wouldn't have imploded in the deep ocean. Their death was caused by the sinking of the Titanic (And a cavalier attitude under water safety).