The Undulatus effect forms due to wind shear that occurs in the stable layer of the atmosphere. The disturbance in the atmospheric layer can be due to the sudden change in wind speed and direction caused by the approach of a frontal weather system or by simple atmospheric convection (uplift of warm air due to surface heating) mountain tops. The disturbance of the cloud layer causes the waves of air to move like waves on a pond. As the winds blow at different speeds through the cloud layer, the clouds pile up into curved ridges, bind together, and split into waves.
There are many potential explanations for sightings. We recommend eliminating the most common and mundane before jumping to less probable conclusions or you submit a report.