The glory of the Lord took the form of fire coming down from heaven and it burned up all the flesh on the altar. While the narrative makes clear that the sacrificial animals had already been placed on the altar, it would have taken some time for the process to complete. The divine fire caused the instantaneous consumption of the sacrificial animals.
24 And fire came out from before the LORD and consumed the burnt offering and the pieces of fat on the altar, and when all the people saw it, they shouted and fell on their faces.