Instant death screen

[Cezbor]

One possibility is: you've reach the crushdepth and you were not "picking up depth" as fast as it ought to be done to stop increasing hull damage;(note that crushdepth is not a steady value. It depends of your hull damage status. Also SH3 commander randomise the crushdepth if you choose this option)
Another possibility: You've been precise depthcharged. Depthcharge could stroke vital point of your sub cousing instant death. It happened to me once and I was really startled .

[Jimbuna]

It sounds like one of the cans in the pattern hit you bang on the button.

Remember, depths can be staggered, they are not always set to detonate at the same depth.

Originally Posted by Cezbor

Depthcharge could stroke vital point of your sub cousing instant death.

This might be it.. direct hit by depth charge..