I've seen it a couple of times and I think it's when the convoy is struggling to turn around a waypoint, usually when I've badly damaged the convoy leader.
Normally, as the convoy reaches a waypoint the convoy leader makes the turn and the rest of the convoy swings around the leader, sometimes leading to some amusing scrambles by ships to assume their old formation places.
I've found that when I'm at high TC at ahead 1/3 when submerged I don't seem to be able to make turns at waypoints and end up driving in circles. I don't know if it's the particular speed that I'm at but it happens almost every time. If the same happens to a convoy then it would cause all of the ships to run around like headless chickens chasing the waypoint marker.
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