The Tiferet Yisrael, a commentator on the Mishnah, gives another possible explanation. We see from the Gemara in Baba Kama (82b) that there were two brothers who were members of the Hashmonean family (of Chanukah fame). These brothers, Aristablus and Hyrkanus, fought each other for the throne of Judea. Aristoblus laid siege to Yerushalayim, where Hyrkanus was headquartered. As lambs were needed for the daily sacrifice and there were none in Jerusalem, the inhabitants worked out a deal with the lamb-sellers outside of the city. Everyday, the Jerusalemites would lower a basket full of gold coins over the wall. In return, the lamb-sellers would supply a lamb, which was then hoisted up. One day, an elderly man outside of the wall advised the sellers to supply a pig instead of a lamb. As the pig was unknowingly being hoisted up, the pig stuck its claws into the wall, and all of Yisrael quaked. From this day until the end of the siege, the daily offering was not brought.
The R"av, also a commentator on the Mishnah, gives another explanation. He explains that the Tamid was discontinued during the period of time when Jerusalem was under siege prior to the destruction of the Temple. The Tiferet Yisrael elaborates on this and says that the R"av is referring to the three year siege of Jerusalem by Nevuchadnezzar, at the time of the destruction of the first Temple. (Torah.org)
[Since the 17th of Tammuz fell on Shabbat, the fast was postponed until today, Sunday July 24th.]
17th of Tammuz: Breaking of the Luchot
