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

Why then Bali got angry with him later?

Bali'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 Bali's position as a king when Bali came back.