Irishlass is right (as usual). My previous microwave did that if the cover wasn't scrupulously clean. Those things are made of mica-impregnated paper and are transparent to microwaves. The problem is that if they get some grease splashed on them, that spot is no longer transparent. The waveguide opening is where all the microwaves come out into the box, so the field density is really high right there. Those now non-transparent spots start absorbing the microwaves and heating up until they get hot enough to start "burning" the mica and its paper substrate.
This isn't a "fault" with the microwave, per se, other than a poor design. (That's why you keep seeing the same problem with different units of the same model, by the way.) If you have one of the designs with the waveguide opening on the side like the one in the video, you essentially MUST use a plate cover to stop oils splashing onto the waveguide cover. Water isn't usually a problem because it will evaporate, and solids will often dry out quickly enough to stop absorbing the microwaves. But oils both absorb microwaves well and stick around long enough for things to get hot enough to cause a problem - with the added bonus of being flammable themselves once things get going.
Luckily those covers are cheap and simple to replace. As long as the metal sides of the box aren't scorched or melted, you can just keep replacing them if somebody (like a kid) makes a mistake and forgets to use a plate cover.
