Microwaves resonate with metallic materials and water. When an object with capacitance, like a tree, is radiated, it discharges the electrons to earth. Just like how you see arcing from metallic objects inside a regular microwave (which is in the 500-1ghz range, somewhere about there), this is from the electrons escaping to get into the earthed body of the microwave.
If you fully encapsulate a metallic object in water, such as an egg wrapped in aluminium foil, it is safe to irradiate the water and the egg will not be directly heated, which causes some nasty reactions that smell really bad, and probably are not healthy.a I mean really bad... but the water soaks the radiation and cooks the egg via regular conduction. Any metallic object inside the body of water will be effectively shielded, as the water absorbs the thermal effect of the microwaves on the metal.
The most microwave-thermal metals are magnetic, ie, iron, cobalt, nickel. The heating effect of microwaves is from the magnetic induction in the material, which flips polarity in phase with the microwave frequencies involved.
Since there is small amounts of iron in most plants, large plants like trees would have enough magnetic induction to heat substantially above the temperature of water and catch fire.
Think of what happens when lightning hits a tree. The core of the tree explodes from heat and this builds up steam which then expands like an explosive.
There has been recorded cases of pressure cookers jamming and causing very substantial, non-fiery explosions that killed people and did a lot of damage. I think in some movie, tv shows and stories this has also appeared, and I know from when I was a kid putting cans of drink in microwaves was pretty messy.
