Teamsters Union supporters weren't wild about what the protestors had to say about the President and Hillary and a melee ensued where anti-Clinton protesters were punched, stomped and kicked. People are sometimes puzzled by union violence, but violence and intimidation are part and parcel of union power and easily understood with some basic economics.
Competition is always between either seller and seller or buyer and buyer.
If you said General Motors and Honda, go to the head of the class. It's surely not employers. The union's struggle, like that of any other seller, is mainly against their competition - other workers. Union rhetoric would have us believe their main struggle is against employers. They'd also have us believe that the strike is their main weapon in that struggle.