Jamieson, Fausset and Brown on 2 Kings 19:35

35. in the morning ... they were all dead corpses--It was the miraculous interposition of the Almighty that defended Jerusalem. As to the secondary agent employed in the destruction of the Assyrian army, it is most probable that it was effected by a hot south wind, the simoon, such as to this day often envelops and destroys whole caravans. This conjecture is supported by 2Ki 19:7 and Jr 51:1. The destruction was during the night; the officers and soldiers, being in full security, were negligent; their discipline was relaxed; the camp guards were not alert, or perhaps they themselves were the first taken off, and those who slept, not wrapped up, imbibed the poison plentifully. If this had been an evening of dissolute mirth (no uncommon thing in a camp), their joy (perhaps for a victory), or "the first night of their attacking the city," says Josephus, became, by its effects, one means of their destruction [Calmet, Fragments].