Your sweet potato caught fire in the microwave because it was likely cooked too long.
Cooking the sweet potato too long can cause the sweet potato to dry out and then catch fire.
Sweet potatoes and other potatoes catching fire in the microwave is not an uncommon problem, potatoes and sweet potatoes should only be microwaved from above 4 to 5 minutes; longer than that, the potato begins to dry out on the inside, carbonizes, and then burns – so there's either a lot of smoke or even a fire.
You can also place a cup of water in the microwave with the sweet potato to help keep the potato moist as it cooks to prevent it from drying out.
Sweet potatoes can also spark in the microwave because of the minerals that the sweet potatoes contain.
The minerals that sweet potatoes contain include iron, magnesium, and selenium which can create an arcing effect in the microwave and cause sparks.
Dense vegetables such as green beans, carrots, spinach, and green peppers contain a higher amount of minerals in them than other types of food.
These minerals - which include iron, magnesium, and selenium – act like tiny pieces of metal and create what is known as an “arcing effect” in microwaves.
When you heat your sweet potato in the microwave, it warms up quickly and steam gathers in-between the fleshy inside and the skin.
If you don't poke holes in the skin for the steam to escape, your potato will explode in the microwave.