When you sub is submerged, you have to keep going to maintain a certain depth. Just like some "primitive" fishes that would go to bottom if they stop swimming (ie sharks).
If I remember correctly, the minimum cruise speed for a submerged Uboot, called schleichfahrt, corresponded to the propellers running at 90 rpm. The boat was then cruising at only 2.5-2.75 kts
Maybe a bug? If your sub is going at 2+ kts it should keep its intended depth.
Or maybe the crew was so depressed (buggy morale system) that they decided to put an end to their misery
