The Talmud (Berakhot 23b, Sukkah 41b) teaches that one should not hold a sefer Torah or tefilin in his arms while he is praying. Rashi comments that the fear is that these items will fall while the wearer is concentrating on prayer rather than on protecting them. The Kappot Temarim to Sukkah observes that this passage can be adduced to support the custom of fasting upon the falling of a sefer Torah or tefilin, a custom cited by the Magen Avraham (O.C. 44:5).

R. Moshe Feinstein (Resp. Iggerot Moshe, O.C, III, 3) questions the need for this proof, asserting that the fact that it is disgraceful for a sefer Torah to
fall to the ground is obvious and gains nothing from this text. The Resp. Chukkei Chaim (Y.D. 77:2) suggests that the Kappot Temarim is stating that one who is guarding a sefer Torah when no one else is available is exempt from prayer; one who chooses to pray nonetheless is negligent and bears full responsibility for whatever happens to the sefer Torah. This view is consistent with the statement of Chazal quoted and interpreted by the Ramban on the verse (Devarim 27:26) “cursed is one who will not uphold the words of this Torah” – “this refers to to an official of the synagogue who doesn’t uphold the sefer Torah to hold it properly so it does not fall.”

The Resp. Divrei Chaim (Y.D. 59) objects to the Kappot Temarim’s proof by noting that nothing in that Talmudical passage hints at an obligation to fast, and noting that the Talmud’s references to tearing k’riah for a sefer Torah that burns (Moed Katan 26a) refers to destruction of the sefer Torah, not merely disgrace, and excludes accidental circumstances. Rather, he cites the passage in Ta’anit (16a) that states that during times of communal distress the sefer Torah was taken out to the street with the declaration “we had this modest utensil, and in was disgraced by our sins”. The implication is that disgrace to a sefer Torah is a result of sin by the Jewish people. If an incident occurs, it is to be interpreted as a sign from above that the Jews must repent (as expressed by Resp. Mahari Bruna, 127). [Note the objection to this proof in Resp. Divrei Yoel, 7.]

Thus it emerges that there are two approaches to the customary fast for a fallen sefer Torah: repentance for the sin of negligence in proper care for the sefer, and general repentance in response to a Heavenly sign in the form of an untoward event involving the sefer. These two perspectives affect many issues within this practice, such as the question of who is obligated in the fast (e.g., the one who drops the sefer, the one who witnesses it, the entire community – see Resp. Maharash Engel, I, 82; Resp. Chaim Sh’al, 12; Resp M’oznayim L’Mishpat, 5); the question of a disgrace that happens under completely unavoidable circumstances (see Divrei Chaim, ibid, and Resp. Shem Olam I; 39); an incident that happens during the performance of a mitzvah (see Resp. Emek Halachah II, 9 and Resp. Imrei David ); a sefer that falls on Shabbat (see Resp. Beit Yitzchak, Y.D., II, 165); on Yom Kippur (see Resp. Siach Yitzchak, 297); and other issues.