Had you taken much previous damage? Are you using any mods? I've had issues with attempting to submerge and suddenly sinking after a certain depth. They talk about certain bugs to do with bulkheads and hull integrity that raises your crush depth in this thread. It might help explain some stuff. I downloaded the die slowly patch by redwine and so far has helped me avoid stuff like this...
Here's the link to the thread.
http://www.subsim.com/radioroom/showthread.php?t=115075