It was god's time of the month.

1) According to the Bible, Lot's wife did not follow the instructions which had been given. " 15 When morning came, the angels urged Lot to hurry, saying, Arise, take your wife and two daughters who are here [and be off], lest you [too] be consumed and swept away in the iniquity and punishment of the city. 16 But while he lingered, the men seized him and his wife and his two daughters by the hand, for the Lord was merciful to him; and they brought him forth and set him outside the city and left him there. 17 And when they had brought them forth, they said, Escape for your life! Do not look behind you or stop anywhere in [a]the whole valley; escape to the mountains [of Moab], lest you be consumed. 18 And Lot said to them, Oh, not that, my lords! 19 Behold now, your servant has found favor in your sight, and you have magnified your kindness and mercy to me in saving my life; but I cannot escape to the mountains, lest the evil overtake me, and I die. 20 See now yonder city; it is near enough to flee to, and it is a little one. Oh, let me escape to it! Is it not a little one? And my life will be saved! 21 And [the angel] said to him, See, I have yielded to your entreaty concerning this thing also; I will not destroy this city of which you have spoken. 22 Make haste and take refuge there, for I cannot do anything until you arrive there. Therefore the name of the city was called Zoar [little]. 23 The sun had risen over the earth when Lot entered Zoar. 24 Then the Lord rained on Sodom and on Gomorrah brimstone and fire from the Lord out of the heavens. 25 He overthrew, destroyed, and ended those cities, and all the valley and all the inhabitants of the cities, and what grew on the ground. 26 But [Lot's] wife looked back from behind him, and she [b]became a pillar of salt." Source and further information: "Genesis 19 (Amplified Bible)" http://www.biblegateway.com/passage/?book_id=1&chapter=19&version=45 2) "28 So also [it was the same] as it was in the days of Lot. [People] ate, they drank, they bought, they sold, they planted, they built; 29 But on the [very] day that Lot went out of Sodom, it rained fire and brimstone from heaven and destroyed [them] all. 30 That is the way it will be on the day that the Son of Man is revealed.(A) 31 On that day let him who is on the housetop, with his belongings in the house, not come down [and go inside] to carry them away; and likewise let him who is in the field not turn back. 32 Remember Lot's wife!" Source and further information: Luke 17:28-32 (Amplified Bible) http://www.biblegateway.com/passage/?search=luke%2017:28-32;&version=45; Further information: http://en.wikipedia.org/wiki/Lot_(Bible%29