We tend to envision Lot's wife becoming a salt pillar as a (lehavdil) medusa-like sudden freezing from a normal human-organic state to a stone-cold hard mineral state.
Was this incident a miracle?
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This clearly implies that the fact that she and the rest of Lot's family were not getting hit by this downpour was a miracle. By looking back at the suffering of S'dom she lost the the privilege of this miracle so Hashem removed His divine protection from her and therefore she was hit.
Lot's wife becoming a salt pillar was a natural event (given the miraculous disastrous circumstances) that was prevented until then by super-natural means.
Ralbag in his commentary to this incident explains that she did not turn into a pillar of salt at all (not miraculously, nor non-miraculously), but instead it was the land which became like a pillar of salt:
ותהי נציב מלח. רוצה לומר שארץ סדום ועמורה היתה כמו נציב גדול ממלח כי לחזק השרפה נתחדש שם מלח כאמרו גפרית ומלח שרפה כל ארצה וראוי שתדע כי מהארץ הנשרפת יתחדש המלח כמו שהתבאר בספר האותות ולא זכר מה שקרה לאשתו בהביטה מאחריו להשענו על מה שזכר המלאך שיקרה לו אם יביט אחריו ועוד שכבר זכר אחר זה שכבר עלה לוט מצוער וישב בהר הוא ושתי בנותיו וזה ראיה שכבר מתה אשתו
And she was a pillar of salt. Meaning to say that the land of Sodom and Amorah was like a great pillar of salt, because due to the strength of the burning salt was generated there in accordance with the verse "and that the whole land thereof is brimstone, and salt, and a burning" (Mechon Mamre). And you should know that from burned land salt will be generated, as is explained in Sefer Ha'otot. And it does not mention what happened to his wife when she looked behind him, relying on that which the "angel" had mentioned would happen to him if he looked behind him. And furthermore, it already mentions after this that Lot had already gone up from Tzoar and settled on the mountain, he and his two daughters, and this is a proof that his wife had already died.
He later reiterates that it was the land, not Lot's wife, that became the pillar of salt:
והנה הביטה אשת לוט מאחריו ונספת עמהם והיתה אז הארץ ההיא כמו נציב מלח לחזק השרפה אשר היתה שם
And behold, Lot's wife looked behind him and she perished with them, and the land was then like a pillar of salt due to the strength of the burning that was there.
And then he reiterates this yet again, and explains why he interprets the verse this way:
ולא הסכמנו גם כן שיהיה אומרו ותהי נציב מלח שב לאשת לוט כי השם יתעלה לא יעשה המופתים אם לא לתועלת שייראו מלפניו והנה לא היה שם מי שראה זה המופת ולזה נראה לנו שאמרו ותהי נציב מלח שב לארץ סדום ועמורה כי בהתהפך המקום ההוא נראה בו גפרית ומלח ולזה היה כמו נציב מלח
And we also do not agree that the phrase "and she was a pillar of salt" refers back to Lot's wife. Because God – may he be exalted – does not perform miracles if not for the benefit that [people] should fear before him, and here there wasn't anyone that saw this miracle. And therefore it appears to us that the phrase "and she was a pillar of salt" refers back to the land of Sodom and Amorah, for when the place was overturned brimstone and salt appeared in it, and thus it was like a pillar of salt.
Thus, according to Ralbag no miracle occurred, because such a miracle would have served no purpose.
A couple of other commentators cite similar explanations, though with less elaboration. Chizkuni writes:
דבר אחר ותבט אשתו מאחריו ותהי כל הארץ נציב מלח שכן גפרית ומלח שרפה כל ארצה
Another explanation: And his wife looked behind him and the entire land was a pillar of salt, for brimstone and salt burned the whole land.
כתב החכם ר' אברהם ן' עזרא ותהי הארץ ועל דרך רז"ל ותהי אשתו
The scholar Ibn Ezra wrote "and the land was [a pillar of salt]", and according to the Sages of blessed memory "and his wife was [a pillar of salt]". (My emphasis)