How could a microwave's magnetron break down due to the presence of metal inside the oven? I have a combined microwave oven/grill (Ikea Granslos) that broke down recently. It looks like this:

There is a wire shelf in the middle that I've always assumed to be compatible with both baking and microwave modes. However recently the microwave's magnetron stopped working and I had the oven serviced. The technician who fixed it claimed that the root cause is due to the presence of the wire shelf and showed me that the manual indeed recommends not to use it.
But how is it possible for the wire shelf to somehow damage the microwave? I can see how it could generate sparks and overheat, but how could this affect the magnetron? The oven is designed for high temperatures, so overheating alone could not have caused it. Or perhaps the wire shelf creates reverse electric feedback that can damage the magnetron over time?
 A: 
Or perhaps the wire shelf creates reverse electric feedback that can damage the magnetron over time?

Yes.
In a microwave oven, the microwave is not just a coil of wire with current going through it. Anything that absorbs the microwaves it emits (be it a spoon, a wire rack, a glass of water, or the water inside a piece of chicken) will absorb it by developing a current (which can be carried by free charges, as in a metal, or a polarization current carried by bound charges, as in water), and this current will emit its own microwave field which will interact with the magnetron; indeed, this is how power gets drawn from the magnetron in the first place.
The bigger those currents, the more the magnetron has to work. So, yes, it is perfectly possible that if the currents induced inside the oven cavity are too big (say, because you left in a piece of metal that was ruled out by the oven's design criteria, as set out in its manual) then the field they emit can cause the magnetron to work too hard and end up getting damaged.
For any specific device, of course, the conditions for safe operation will be specified in the manual.
