because they're extremists.
because they're extremist rightists who dress up as leftists to make everyone believe leftists are causing the trouble.
because they are neither and see it as a good excuse for a punch up.
there are government agents who infiltrate the groups to stir up violence.
there are rightists who infiltrate the group to stir up violence.
take your pick, there are probably loads more choices.
splitting society into a leftist/rightist division is implementing the divide and rule arguement, when you ask questions using those terms you are perpetuating the illusion.