It is explained that Vali (also called Bali in North India) was a proper and powerful king. In the beginning also, there is no incident of rivalry between Sugriva and Vali. While chasing Dundubhi demon, Vali told Sugriva to wait outside. Sugriva did only what he was told.

But why did Vali get angry with him later?

Vali's anger is not justified since he was a proper king. I also find something missing because he is not supposed to get angry because Sugriva accepted Vali's position as a king when Bali came back.