John Calvin Commentary Numbers 16:35

"And fire came forth from Jehovah, and devoured the two hundred and fifty men that offered the incense." — Numbers 16:35 (ASV)

And there came out a fire from the Lord. The diversity of the punishments had the effect of awakening more astonishment in the people than if all had been destroyed in the same way, although God’s anger raged more fiercely against the original authors of the evil, making it manifest that each received a recompense according to the measure of his iniquity. He says that a fire went forth from Jehovah, because it was not kindled naturally, nor accidentally, but was accompanied by conspicuous marks, which showed that it was sent by Him. Yet I do not reject the opinions of others, namely, that God thundered from heaven, since in this way His power would have been more manifestly exerted.