The multipactor effect is due to electrons that are accelerated by radio-frequency (RF) fields and create an electron avalanche with secondary electrons emission. The number of secondary electrons depends on the primary electron energy and its impact angle.
The acceleration by the RF fields induces even more impacts with surfaces and the quantity of electron can grow exponentially and lead to operational problems. When RF go through waveguides, it can rip off electrons from iris surfaces.
These electrons can have sufficient energy to cause the release of secondary electrons leading to a cascade of electrons called the multipactor effect.