‘And it happened that, when they had brought them out, he said, “Escape for your life. Do not look behind you. Do not stay in all the Plain. Escape to the mountain lest you be consumed.” '
The ‘he' has been prepared for by the phrase ‘Yahweh being merciful to him'. He is now directly aware of the voice of Yahweh. Probably there is too a theophany of some kind, possibly in the form in which Yahweh had appeared previously in chapter 18. Lot now knows he is not just dealing with angels. Yahweh is involved. The message is clear. The whole plain of Jordan is to be destroyed. The mountains are the only place of refuge.