On Sukkos, there is a custom to perform a cermony called Hoshanos. During this ceremony, the congregation parades around the bima while holding their lulavim and esrogim. There are two opinions on when this should be done: 1) immediately after Hallel or 2) after the Torah reading and Musaf. The reason some people do it immediately after Hallel is because during that time, the Lulavim and Esrogim are held. To give honor to the mitzvah, Hoshanos are done then so the Lulavim and Esrogim are held longer. The reason other people recite Hoshanos after the Torah reading and Musaf is because Hoshanos is a minhag (custom), while reading the Torah and Musaf are more than a custom.