I am not a submariner, but one scenario I can envision is that our sub was shadowing and recording the sounds of another sub. The other sub, having no clue that we were even there, proceeded to learn otherwise by blundering into ours.
I am positive that our guys knew the bearing and range of the other, but have no idea if they can pinpoint depth accurately enough to know if an approaching sub would pass under or over them by a few feet. Maneuvering quickly to get out of the way would likely reveal themselves, which is a choice they may have wished to avoid. And I will not surmise either way.
Fighter pilots say "The first one to light up dies." That refers to radar or afterburners, but I'd bet the same thing applies to subs in different ways.
