The frequency is probably just an easy one to build magnetrons for.

The real reason is that that range is reserved for consumer devices so that it doesn’t interfere with actual ISM sanctioned communications as enforced by the FCC. We just also decided to put wifi in the same range cause they’re stingy releasing frequencies for public use.

https://en.wikipedia.org/wiki/ISM_radio_band#Frequency_allocations

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But research was done on it cause of course it has been.

https://iopscience.iop.org/article/10.1088/0031-9120/39/1/006

This article deals with the generation of microwaves in the oven and includes the operation of the magnetrons, waveguides and standing waves in resonant cavities. It then considers the absorption of microwaves by foods, discussing the dielectric relaxation of water, penetration depths of electromagnetic waves in matter and, in considering the possible chemical changes during the microwave heating, multi-photon ionization or dissociation.

So you’re likely right that it’s not water resonance, but chassis cavity resonance. I can’t say that I’ve read deeply into it. And thinking about it I remember hearing something about some of the high level stuff that I just read in relation to this article. I probably ran into it in passing and just failed to recall it. But to be frank, I’m okay just calling it voodoo wizardry in of itself. But I have to understand wireless communications stuff for my profession, and it’s well known that it’s basically the same range as wifi 2.4ghz/bluetooth/other consumer standards that sit in the same crowded space.