The "root" cause of this "phenomenon", aka: "design feature" is improperly terminated Random Generated Groups in the Jap_HarborTraffic.mis file, that are supposed to be "Docked" ships with a zero speed. They were instead accidentally left at the default speed setting of 5 knots. This is another reason why we split that file into "layers" for FotRSU.
The ships sail around the world - literally - with no set course, just their initial heading, and since they are only in the database until the player's sub gets close enough to spawn them, they do not care about land masses or other obstructions, so they keep on sailing on that same heading, traversing the globe. Every few days a new "group" (generally one ship) will spawn, and they will gradually build-up over the "years" in the game into those massive DD convoys, of which off Honshu could be parts of 4 or 5 such groups. I think there is like seven sets of them in the Stock game. Some mods actually made the situation worse by copying those groups and placing them elsewhere in the "world".
Another aspect that can contribute to the situation is if the player adds and subtracts mods from the game that alter the campaign files, and does not empty the Save folder between such mods. In cases like that, one set-up might have a given group moving, while the next has the same-named group stationary. Not clearing the Save data can then cause a bad mix of the two mods, resulting in a bunch of Flying Dutchman type ships, doomed to sail the seas forever...
