I replied to another thread about this not too long ago, but cannot find it just yet, but we named the massive DD groups as "run-a-way spawns". There are two ways those can happen, with the main one being the use of certain mods that have improperly configured RGG (Random Generated Groups) that appear to have been intended to be "docked" in a harbor, but the default speed was left at 5 knots, and the "DockedShip" was not set. There are no waypoints on the RGG, so they take off at 5knots in the direction they are facing. The game then runs the "around the world", and they come back on the other side of the SH "tube" world, eventually turning into "groups" of DD in columns as time progresses. They will eventually take the game down due to "bandwidth" if you encounter them. Just too much for the game engine to have hundreds and hundreds of ships in one scene.
The other ways to get similar to that is to either shell-out to Windows (<Ctrl><Tab>, <Alt><Tab>, or <Windows> key combinations), or by not deleting the Save folder contents after changing mods, both of which can and do result in invalid data in the game and corrupted Save files.
