In Leviticus 9:24, God Himself lit the fire. And because of this, it became holy fire, as though the Hand of God touched it. It was never to be allowed to go out; it had to be fed constantly to keep it from going out. The fire of God was always with them continuously in the pillar of fire above and below. They carried it with them wherever they went.
No one knows for sure exactly what occurred, but somewhere on the outskirts of the camp of Israel a literal fire broke out. Whether it was started by lightening or fire out of heaven, nobody knows, only that it was from the Lord. The fire began to consume part of the camp. Soon, people began to experience pain as a result.