Same thing happens to me. My subs have the enemy sub in range, but it turns around to target the destroyers/first row ships, and my front line ships will have to deal with the enemy sub, getting sunk by the second line ships before they can fully sink it.
This is because the Mogami has a certain amount of torpedo launchers on one side. Take the Atlanta for an example. It has 2 launchers, 1 on each side. So, it can't fire both at once unless you don't move the launchers and press the launch button. This is the same for the Mogami. You can fire your first set of torpedos, then turn to the other side to fire the other 2