When Vali threw Dundhubi's corpse into the air, it flew over the ashram of the rishi Matanga on Rishyamukha Hill.
Drops of Dundhubi's blood rained down upon the ashram, and this made Matanga angry. When he saw Dundhubi's corpse lying on the ground, he shouted out a curse. "Whoever threw this corpse will die if he ever sets foot on Rishyamukha Hill. His head will explode in a thousand pieces!"
So when Sugriva later had to flee from Vali, he hid on Rishyamukha Hill.
Why did Sugriva have to flee his brother Vali?
It was because of Dundhubi's son, Mayavi.
Inspired by: Valmiki's Ramayana
Notes: This is the Kishkindha Kanda (Book 4), sarga 11.