The Fast Day Torah Reading
At both the shacharit and mincha service on fast days (only at mincha on Tisha B’av), the theme of the Torah reading is the sin of the Golden Calf. During the course of this reading there are several verses that are first recited out loud by the congregation and then repeated by the ba'al koray. This happens during the Kohen aliya with the verse: "shuv m'charon apecha…" and then again in the Yisrael aliya with the verse: "Hashem Hashem…" and the verse "v'salachta l'avoneinu."
In the Levi Aliya, however, there is no such verse. There is no interaction. There is no audience participation.
Why the difference?
It is explained that the reason there is no “interaction” in the Levi aliya is because it was only the Kohanim and the Yisraelim who participated in the sin of the Golden Calf. It was Aharon, the Kohen Gadol, who actually called for and fashioned the Golden Calf and it was the Yisraelim who worshipped it. The Leviim, however, did not participate in the sin in any way. Therefore, there is no need to emphasize any of the verses of admonishment during the Levi aliya. Indeed, we are taught that the emphasis and repetition of the verses of admonishment during the other two aliyot is intended to embarrass the community for the sin committed by their ancestors as well as to stir them (along with the Leviim) to repentance.
See also Sefer Kushiot #85, Sefer Chassidim #250, Yosef Ometz #47, and Kitzur Shulchan Aruch 78:4 for more on this custom.