I guarantee that they can cause fires with correctly sized magnetically susceptible particles of sufficient size, and sufficiently high energy levels.

You would be familiar with how a coathanger wire gets hot when you bend it. The changing of the geometry of metals opens up gaps in their semi-crystalline structure that heat escapes from. Microwaves induce this on a very small scale, in a conventional microwave it's around 2-3x the diameter of the faraday cage mesh in the front door, for a millimetre wave, obviously it's 1/10th or so this size, getting smaller as you go up the spectrum until you get to infrared.

Really, photons and electrons are the same thing just that photons have wavelengths that are small enough to directly interact with molecules and atoms.

But as for how they cause the fires, my guess is it's not a direct effect, but rather that there is bits of metal around, especially the soil, the microwaves heat them up until they emit enough infrared that nearby, dry plant material like dead grass or eucalyptus leaves, especially those, since they are full of flammable oils, have enough kinetic energy to start reacting with the oxygen in the air, and voila, fire.