You didnt specify what type sub you had. Even with a gato you would be fubar'd at 66% - thats 2/3 hull damage. Given the game figures a gato is safe at say - 350 feet, 66% of your hull integrity is gone - meaning you have given up about 231 feet of max dept. That leaves you able to "safely" dive up to 119 feet. You went deeper than that. Anytime your deeper than your "safe depth", additional hull damage accrues - but never gets called out by your crew. It continues to accrue until you SURFACE - meaning that even if you come above your safe depth - the damage is still happening until you surface - which you didn't. So as you manuevered, your hull integrity continued to drop, and when you tried to duck under the layer, your sub finally "broke" under the strain - taking you down to crush. While it may at first appear that its a bug, its actually a decent (though not perfect) way to simulate the stress on your boat.
Hope this explanation helps you understand that what happened is simply because you choose to take your boat below what its damage level defined as safe. Had you never gone deeper than ~120 feet, you would have been ok, provided you could still lose the escorts.
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Good Hunting!
Captain Haplo
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