Hi Jesse,
A torpedo can do considerable damage to the hull of a ship, take the Lusitania for example. A large underwater mine can do far more damage to a larger area of the ship.
As Michael said, there was nothing to cause a second explosion and I do not think any bulkheads collapsed during the sinking.
I place the rapid sinking on the large amount of damage done by the explosion, and that the ship was moving at Full Speed while sinking, forcing more water into the hull that would have ordinarily have entered had the ship stopped immediately - more water than even the additional bulkhead caps could have stopped.
The Britannic would probably still have sank considering the fact that a high percentage of windows and portholes were open, there is a slight possibility that she would have remained afloat had she stopped immediately, and could possibly have even been patched and towed to port for repairs.
Best Regards,
Brian