What is the primary cause of loss in film capacitors at RF?

Explication

The pool's key is skin effect: as frequency rises the current crowds into the surface of the foil or metallization and of the terminations, so the electrode resistance and with it the equivalent series resistance grow. In a real film capacitor the RF loss is a sum of electrode resistance, contact and end-termination resistance, and dielectric loss, and which term dominates depends on the film, the electrode construction and the frequency; skin effect is the mechanism the pool has in mind, not a universally established ranking.