I don't mean that it had a crack in it, but that it developed one as a precursor to total failure. My experience of physical materials is that they almost never just atomize, rather there's a weakpoint from which the failure cascades. I'm envisioning a process that might only last a few seconds, or be measurable in milliseconds.

The forces are immense and act all the time. In your daily life, you don't usually have that kind of system, so this feels counterintuitive.

Press against a solid object gently with one hand on each side, then suddenly yank away one hand. The object will be accelerated gently. Now do that with all your strength and send the object flying through the room in no time, with a few dozen kg of pressure. Now imagine that you could apply hundreds of tons of pressure. In such a system, every change will be nigh instantaneous.

I understand the physics just fine, thanks. I'm not disputing that it will be 'nigh instantaneous', I'm talking about how it fails. You and downvoters are also neglecting the fact that the submersible isn't suddenly teleported to/near its destination at which point it suddenly fails. It gets there slowly, passing through its safety envelope on the way.