The Gemoro says that lines of kohanim were used to pass the blood of Korban Pesach towards the mizbeach because of B'Rov Om Hadras Melech. I assumed before this that it was somehow to save time. If the reason is B'Rov Om, why was this not done on other occasions? (Maybe it was?) (e.g. On Shabbos when there were not many Korbonos, all Kohanim could have formed a line to pass the blood).
The only thing I could think of is that on Erev Pesach when there were so many Korbonos to be brought in a limited time, there would be many Shechita at one time, and there were many lines going in to the mizbeach, and this was the B'Rov Om. (But I don't see that it answers very much, because why should a single line not be B'Rov Om?.
See Yoma 26a and Tosfos there, DH Makom Shechinah. It would seem that when the presence of an extra Kohen is totally unnecessary, everyone agrees that it looks like the Kohen does not want to do the Avodah if he passes it on in the middle. On Erev Pesach, though, when many Kohanim are needed anyway, there is no problem, and the Machlokes between Rebbi Eliezer ben Yakov and the Tana of the Mishnah applies where the Avodah seems complete if we can add additional Kohanim.
D. Zupnik