Why did Lot's wife turn round, despite the warning not to?
Targum Yonasan: To see what was happening to her family, since she was a Sedomian.
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 to her angelic visitors on the previous night. 1
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 [Pirush Na'ar Yonasan]).
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.