Because they're beating up the street protesters, who are his found family.

Also revenge for the hundreds of dogs cops shoot every year why not.

Might want to watch this video to make sense of anarchism. Might give you a slight idea about the issues with statism.

Cops are the legalized thugs of the state, the violent arm of private property. They are the natural enemy of all active left organizers, being the people that will actually shoot you when you do work against capital (including against cops themselves). This runs a wide range. They break up strikes by union workers. They shoot protesters. They arrest those who do direct action against particularly oppressive projects like the Keystone Pipeline XL. They harass anyone that looks a certain way. They enforce evictions.

Matapocos was a street dog that attended protests in Santiago Chile and bit cops because they attacked protesters. He was reportedly then semi-adopted and taken care of by left organizers.