I’ve heard this phrase being thrown around sometimes on other game tables. Apparently, it describes a straight line of melee combatants on a grid, alternating between members of two opposing groups. This only occurs if the Flanking Variant Rule are used. Under these circumstances, the Conga Line of Death makes sense from a mechanical standpoint, as every melee combatant wants to get advantage on their attack rolls. If this rule were in place, player characters as well as NPCs would understand the in-universe ramifications and probably try to get into a superior position every time, leading to the aforementioned Conga Line of Death.
Now my question is: How can I, as DM, avoid the Conga Line of Death occurring when implementing some form of flanking rule?
The obvious answer would be: “Don’t use the Flanking Variant Rule”. Well, I for one like to grant some form of advantage, when two combatants gang up on their opponent. And it kind of makes sense, that it is easier to hit somebody who has to avoid the attacks of two enemies. So I want to keep Flanking in my game, but like to prevent the Conga Line of Death.