I also think that to learn Musar on Rosh Hashanah is important. Also that even though it is unspoken, it must be  that Reb Israel Salanter must have had in mind the fact that strange an foreign trends were taking over the world in general, and the world of Torah  in particular.   So he saw Musar as a cure for the evil doctrines that were taking over in the world of Torah. [''Musar'' refers to the books of ethics written mainly during the Middle Ages]