When would they use the torpedoes in the after room?

One reason would be that the target made a sudden course change (zigs or zags) and it became much easier to turn the boat to bring the after tubes to bear than to swing the bow that far around.

However, the captain also needs to remember to use those torpedoes sometimes, regardless of the situation. One-third of the available weapons were back there. Although it was probably a bit easier to plan the attack using the bow tubes, there was one distinct advantage to using the after tubes. As they were firing the torpedoes, they were already headed away from the target and beginning their escape.

One other factor could sometimes be subtle pressure from the crew. Using the torpedoes in the after tubes can improve the bunking situation. When the torpedo in the tube was fired, the reload was moved from the rack into the tube. In the after room, they could convert the empty torpedo rack or skid into bunks. Each skid could be converted into three bunks. When that was done, nine fewer sailors had to “hot bunk.”