the cavity is asymmetric, has EM waves bouncing inside and isn't a Faraday cage - thus generates radiation outside (by Maxwell's classic theory, not virtual particles QM), asymmetric at this. EM radiation, ie. photons, have impulse, so according to Newton's and impulse preservation laws the EM drive should have an impulse in the direction opposite of the total vector sum impulse of that outside radiation.

The impulse is larger than the same energy would generate using visible light frequency photons in photonic drive or solar sail - that is obvious too because they use radio frequencies in EM drive, ie. much lower frequency photons. The same basic math works here like the one for specific impulse, thrust and energy relations in the classic situation of high by-pass jet engine vs. low by-pass vs. rocket engine.

Btw, for current working example of IR frequency photonic drive look up the slowdown of Voyager by IR radiation from its RTG bouncing off the back of its antenna (which is pointed to Earth/Sun and the RTG is behind it thus that IR photons bounce slows the Voyager)