Why do people call cops pigs?

I don't get it. Like where did that come form? I mean cops rock. The are patient,and protectvie. I love them. They are so great. I place them high on my list of people I love. Why would people degrate them by calling them names like this and insulting their authority?
Update: wow deep stuff.
i hate hearing people insult cops. and i heard a 5 year old say they were pigs yesterday
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