Why did harry attack mundungus fletcher in hogsmeade village?

Harry attacks Mundungus because he has seen Mundungus was selling old black family inheritances and heirlooms from Sirius's family house, and headquarters of the Phoenix. Harry was still misery over the recent death of his Godfather who had only later come back into his life after helping 13 years in the magical work of a wizard in the prison Azkaban under a fake charge. Harry was evidently hurt after seeing Mundungus selling Sirius's assets, who had obviously used Sirius's death as his chances to take from the house and sell the goods unlawfully.

Political forces often influence public opinion.