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
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