Why did Lot's wife turn around, despite the warning not to?
Targum Yonasan: To see what was happening to her family, since she was from Sedom.
Behind whom did she peer?
Rashi and Ramban: Behind Lot, who had gone to the back, to hurry-up the members of his household.
Targum Yonasan: Behind the angel.
Why did she turn into a pillar of salt?
Rashi: Because she refused to give a little salt 1 to her angelic visitors on the previous night. 2
Targum Yonasan: Because she went round to all her neighbors that night, asking for salt for her visitors, in order to publicize the fact that guests had arrived, and prompting the people to come and take action (which they did [Peirush Na'ar Yonasan]).
Gur Aryeh: Otherwise, how can we explain why she received this particular punishment?
Interestingly, Chazal equate salt with Chesed (See Kesuvos 66b) [of which Sedom and Lot's wife were the antithesis]. Perhaps that is because salt is in itself unpleasant to eat, yet it renders most other foods edible and tasty. Also refer to 1:21:2.5:1.
QUESTIONS ON RASHI
Rashi writes: "His wife looked behind him - [I.e.] behind Lot." What does Rashi mean?
Mizrachi: If the verse intends that she looked at Sedom, which was behind both of them, it should say, "behind her." Rather, the verse intends, "His wife, who was standing behind him, looked" - without specifying what she looked at.
Gur Aryeh: The previous verse's subject (19:25) was the angel (as he overturned Sedom), but we cannot explain that she looked behind the angel.
