FWWC: Night Movement

[Mowgli]

Most likely this. If you move a unit in travel mode along a road and the "road stacking limit" (see parameters, F4) in the destination hex is already exceeded (by friendly troops already present in the hex) or gets exceeded by the arrival of the unit, then the movement does not count as a road-movement, in which case the unit has to pay the full terrain cost and may disrupt during night turns. It's a chaotic night time traffic jam.

In Serbia, the road stacking limit is about 2600 men, or max 2 batalions + small stuff.