This phenomenon is well known in the industry and has a name: "String lining". Light cars with, normally, heavy cars trailing on sufficient curvature will result in the light cars attempting to shrink the distance (via derailing) much like pulling on a string that was in the shape of a curve will straighten the string out. There is no need for trailing heavy cars if the curvature is very sharp, such as in a yard or on industry tracks, for string lining to happen.
Take care,