Just recently I have had a few phone calls from customers who have damaged their microwave by using it to warm plates.
In a nutshell, don't use your microwave to warm plates.
Very briefly, microwave ovens work by emitting microwave energy into the oven. This in turn agitates water molecules in the food or drink in the oven. The friction of those molecules creates heat and - hey presto - hot food is the result.
Plates have very few if any water molecules in or on them, therefore if you only put plates in the oven, the microwaves will bounce around the oven looking for something to absorb them. Eventually, something else in the oven will get warm. it might be the plastic roller ring, the mica waveguide cover or any plastic parts of the inner door.
It may be that you have been doing this for a while with no problem, this could be due to a number of factors such as the quality or type of plate. Some cookware may have moisture inside which more readily absorbs the microwaves, whereas some cookware will be completely invisible to microwaves (this wont get warm at all).
On a few occasions problems have occurred where the microwave is a replacement microwave and is either much more powerful than the last or through its design is less tolerant of microwave energy bouncing around inside.
Either way, if the microwave is used for heating plates eventually some damage will occur. The plates could break. The microwave oven might even end up cooking itself ultimately which will of course result in costly repairs or replacement.
If you really want to use your microwave to heat plates try putting a bowl of water on top of the plates so that most of the energy will be absorbed or if your microwave is a combi oven warm the plates with the grill of convection instead. It may take 5 minutes instead of 1 minute but wont affect the microwave in any way.