It's difficult to tell from the article, which offers several theories. The first paragraph suggests that microwave exploded, giving him a shock. The fifth paragraph suggests that he was electrocuted. Forensic investigators are still investigating. The usual online expert claimed "The high-voltage microwave oven may have charged something intentionally left inside and caused the explosion" which I presume means he left a hand grenade in the oven while preparing the popcorn. Another suggested that he was murdered by his wife and then blamed the microwave. For all I know, it could have been the vacuum cleaners fault.
I prefer to believe that he was killed by the popcorn. Popcorn is normally quite safe prepared in a microwave oven, as long as each kernel pops at a different time. However, if all the kernels were to pop at exactly the same time, they might produce a steam explosion. Statistically, that's very unlikely, but still possible.
This is what happens when I cooked a yam for 16 minutes instead of 6 minutes: There was no explosion, but there was plenty of fire, smoke, and stench. When I opened the door, the inside of the yam was glowing dull red.
Why do microwaves explode?
Good questions. I don't have an answer. Explosive disassembly normally disconnects the wires that might present an electrocution hazard. In this situation, explosive assembly (with burns) is very unlikely.