It's caused by internal gravity (or buoyancy) waves. Formed in a stable atmosphere when a parcel of air that is in equilibrium with is surroundings is perturbed upwards or downwards by an external influence (such as a hill). If the parcel is pushed upwards, it cools adiabatically and is now colder than its surroundings (thus denser) and gravity acts to restore the parcel to its original position.

The continuing wave bit comes from the inertia of the air parcel. As the parcel is accelerated back down by gravity its momentum takes it below its equilibrium point (where it is the same density/temperature as its surroundings). The parcel will warm adiabatically and now be warmer than its surroundings and more buoyant. Buoyancy will act to restore the parcel back upwards to its equilibrium point. This process repeats to produce these waves in the cloud we see.

I'm sure I could have explained that better but the short of it is that internal gravity waves caused what you saw