It's a bad idea to run your microwave empty.
Microwave energy excites water molecules, a microwave oven heats food by heating the water inside it. If there's no water to excite then the energy ends up heating the components of the microwave instead, which is why it heats up when you run it empty, doing this can damage your microwave.
Popcorn is one of the tougher things you can cook in a microwave because it is very dry, so the microwaves again have little to work with and can end up heating the microwave instead. A popcorn kernel pops when the pressure of the expansion of the small amount of water in the kernel exceeds the ability of the skin to contain it, and when it fails the inside explosively expands, releasing some super-heated steam. The heat of the kernels and the steam released from the popping can heat up your microwave quite a bit, especially if you put too much in. 1/4 to 1/2 cup of popcorn is the maximum I've seen recommended.
Try heating up a full bowl of water for a 2-3 minutes and see if your door heats up until it's dangerous to touch. If it does then your microwave is faulty, if not then your microwave is probably fine and you need to understand its limitations better.