It's been a couple of years since I've seen that movie but this was my take. He took a glass and added a liquor with a high alcohol content. That is a flammable/explosive. Also put in a brillo pad or scrub pad with soap in it. If that soap had any ammonia, it may have contributed to the blast. Then put mixture in microwave and set delay timer. As in, I'm leaving for work now and I want my lunch hot when I walk in the door. Standard feature on most electrical cooking appliances with timers. The beeping before the blast was the microwave starting, not stopping iirc. Have you ever put any metal such as foil or a utensil in a microwave accidentally? It gets very hot very quickly. Wire mesh like a brillo or steel wool scrubber increase the surface area and decreases the heat sink rate so it probably got red hot in one second. I'm not going to ruin my microwave finding out. Commercial microwaves are much more powerful that home models. So, now we have a very flammable mixture under a powerful microwave beam causing it to get hot and fill the atmosphere with flammable vapors.
We have a metal getting red hot and combusting in those vapors. If you doubt a 600+ watt microwave is powerful enough to to do this, take a 1.5 volt flashlight battery and touch a piece of steel wool to both ends. The fireworks should convince you. This is an old trick campers use to start fires. And that is an old trick movie commando's use for those kitchen shootouts. Sheesh. You guys should watch more movies.