A certain young man of the people of that city saw them, and he ran and told all the men of that city, and they all gathered together at the door of the house to do according to their wont, even deeds of sodomy, as it is said, "And they called unto Lot, and said unto him, || Where are the men who came to thee to-night? bring them forth unto us that we may know them" (Gen. 19:5). What did Lot do? Just as Moses gave his life for the people, so Lot gave up his two daughters instead of the two angels, as it is said, "Behold, now, I have two daughters" (Gen. 19:8). But the men would not agree (and did not accept them). What did the angels do to them? They smote them with blindness until the dawn of the (next) morning. All were treated with (measure for) measure. Just as he had taken them by the hand without their will and taken them into his house, so they took hold of his hand, and the hand of his wife, and the hand of his two daughters, and took them outside the city, as it is said, "But he lingered; and the men laid hold upon his hand" (Gen. 19:16). And they said to them: Do not look behind you, for verily the Shekhinah of the Holy One, blessed be He, has descended in order to rain upon Sodom and upon Gomorrah brimstone and fire. The pity of 'Edith the wife of Lot was stirred for her daughters, who were married in Sodom, and she looked back behind her to see if they were coming after her or not. And she saw behind the Shekhinah, and she became a pillar of salt, as it is said, "And his wife looked back from behind him, and she became a pillar of salt" (Gen. 19:26).

A Midrashic interpretation: The reason the angels forbade Lot to look back was that he would see the presence of G’d as it was pouring sulphur and brimstone on the valley. Such a vision would prove fatal for him. Irith, Lot’s wife, had feelings of compassion for her married daughters whom she had left behind, and this is what caused her to turn around. She wanted to see if these daughters now followed them. She beheld the שכינה instead and turned into a pillar of salt.

And Edah, the wife of Lot looked around to see the destruction of ‎the city, for her compassion was aroused for her daughters that did not go along with her but ‎had remained in Sodom. And as she turned backwards to see what hath occurred, she became ‎a pillar of salt. And that pillar is still standing in its place to this very day.

The phrase "(ותבט) אשתו" (and his wife looked back) in gematria is equal to "היא עירית" (she turned into a pillar of salt).

ותבט אשתו מאחריו, “His wife looked behind him.” According to Ibn Ezra, the suffix in the word מאחריו refers to Lot. Rashi claims that the suffix refers to her having ignored the angel’s command. In Pirkey de Rabbi Eliezer it is said that the angels said to both Lot, his wife and daughters, not to look behind, as the Presence of the Lord raining sulphur and fire on the Sodomites was descending on earth immediately behind them. Lot’s wife Iddit, who was concerned about the fate of her married daughters, ignored the instructions, and, as a result, was turned into a pillar of salt. According to this version, the masculine suffix in the word מאחריו would refer to the שכינה, the manifestation of G’d’s presence.

It is written (in Job 20:23): LET IT BE FOR FILLING HIS BELLY, LET HIM CAST HIS BURNING ANGER AT HIM, AND LET HIM BRING DOWN RAIN UPON {THEM} [HIM] IN HIS FLESH (LEHUM). The retribution which came upon the Sodomites was for filling their belly, because they had filled their belly with transgressions, violence, and plunder. Therefore, when retribution came upon them, it filled their belly, as stated (ibid.): LET HIM CAST HIS BURNING ANGER AT {THEM} [HIM]. R. Me'ir said about the destruction of Sodom: The Scripture says (ibid.): AND LET HIM BRING DOWN RAIN UPON {THEM} [HIM] IN HIS LEHUM. (Gen. 19:24:) THEN THE LORD RAINED DOWN UPON SODOM. What is the meaning of IN HIS LEHUM (rt.: LHM)? < That it happened > because of the wars (rt.: LHM) which they had fought with the Holy One. Thus it was stated (in Gen. 13:13): NOW THE PEOPLE OF SODOM WERE EVIL AND SINFUL AGAINST THE LORD, EXCEEDINGLY SO. < They were > EVIL to each other AND SINFUL in adultery, AGAINST THE LORD in idolatry, EXCEEDINGLY SO in bloodshed. R. Judah b. R. Shallum the Levite said in the name of R. Johanan bar Nahman: When Amraphel and his associates came to fight with the Sodomites, because they had captured Lot, Abraham went out to fight against them. So he killed the kings, and they fled, just as it is written (in Gen. 14:10): NOW THE VALLEY OF SIDDIM WAS FULL OF BITUMEN PITS; [AND THE KINGS OF SODOM < AND GOMORRAH > FLED] < AND FELL THERE >. Abraham began reflecting: The kings came to fight with the Sodomites. So the kings fell, but those < of Sodom and Gomorrah > fled. Thus it is stated (ibid., cont.): AND THOSE WHO REMAINED FLED TO THE MOUNTAIN. Perhaps you will say: There were worthy ones among them. The Holy One said to him (in Job 20:24-26): SHOULD HE FLEE FROM THE IRON WEAPON, < THE BOW OF BRASS SHALL PASS THROUGH HIM >; HE DRAWS IT OUT, AND IT COMES OUT OF HIS BODY … UTTER DARKNESS IS LAID UP IN RESERVE FOR HIM; < AN UNFANNED FIRE SHALL CONSUME HIM; IT SHALL BE BAD FOR A SURVIVOR IN HIS TENT >. UTTER DARKNESS IS LAID UP concerns Sodom, since it lies to the south of the land of Israel. Thus it is stated (in Ezek. 16:46): AND YOUR YOUNGER SISTER [THAT DWELLS SOUTH OF YOU IS SODOM AND HER DAUGHTERS]. What is the meaning of SHALL CONSUME THE SURVIVOR IN HIS TENT? That, even if a single survivor is left {of them} [from there], the evil of evil < SHALL CONSUME THE SURVIVOR > IN HIS TENT. Which < survivor > ? This is Lot's wife, Edith, of whom it is stated (in Gen. 19:26): BUT HIS WIFE LOOKED BACK, AND SHE BECAME A PILLAR OF SALT.