I've seen documentaries with models of the ship going under, and all that stuff, and they're pretty accurate and such, so I believe the results they find from that is true. Also, I think the weight of the stern was too heavy for the air inside to keep it on top of the water. There also had to have been several places above water for the air to escape, and the pressure that the heavy stern put on the air forced it out. And while it was going to the bottom, the water getting trapped in this "Cross-Section" of the ship caused it to rip apart, therefore allowing more places for water to get out.