Genesis 19: 26: But his wife looked back from behind him, and she became a pillar of salt.
Lot’s wife disobeyed the LORD'S instruction; she looked behind her to see the destruction falling upon Sodom and was thus transformed into a pillar of salt (Genesis 19: 17: And it came to pass, when they had brought them forth abroad, that he said, Escape for thy life; look not behind thee, neither stay thou in all the plain; escape to the mountain, lest thou be consumed.).
The real reason why Lot’s wife turned to look upon Sodom as it was being destroyed was because she had daughters that had remained inside the city.
Genesis 19: 14: And Lot went out, and spake unto his sons in law, which married his daughters, and said, Up, get you out of this place; for the LORD will destroy this city. But he seemed as one that mocked unto his sons in law.
Genesis 19: 30 - 31: And Lot went up out of Zoar, and dwelt in the mountain, and his two daughters with him; for he feared to dwell in Zoar: and he dwelt in a cave, he and his two daughters. And the firstborn said unto the younger, Our father is old, and there is not a man in the earth to come in unto us after the manner of all the earth:
Lot had daughters that were married and daughters that were unmarried. The married daughters stayed behind in Sodom with their husbands, but the unmarried daughters (they had not received seed) came out of Sodom with Lot.