Milt, pure speculation.
With the advent of modern aerodynamics and wing tip add-ons perhaps the effect is due to an unusual interaction between the wing and engine exhaust.
The effect would then depend on aircraft type / add-on.
I noticed another oddity recently with what appeared to be an A380. This type quickly forms two trails from the four engines, suggesting a wing effect. However, as a one off, two additional faint trails formed apparently below the two main trails, somewhat like an inverted rooster tail. From a distance the effect was similar to fighter aircraft tanking, but there was only one aircraft.
A consideration for both effects is a flight path at the boundary of sufficiently different air masses so that an aircraft wake could actually form cu cloud in your instance, or a stratus like trail in mine.
P.S. Yours?
https://www.google.co.uk/imgres?imgu...CwwLA..i&w=900
Mine were similar to this twin jet.
https://www.google.co.uk/imgres?imgu...AowCg..i&w=700
P.P.S also see
https://cloudappreciationsociety.org/november-2016/
Note that all of the photos are associated with a cloud layer, may be relevant.