Film director James Cameron has expertise in designing and testing these submersibles, and he has many criticisms of the design of the sub that imploded, and of the hubris of the CEO who ignored repeated safety warnings from the diving community. He also mentions that the sub seems to have been attempting to resurface when it imploded, suggesting that they were aware the hull was starting to fail.
The reports I heard said communication was lost when the sub was at a depth of 3300 meters, not feet. But yeah, carbon fiber seems like a bad choice and the thing was an implosion waiting to happen.