Why are the police often called pigs?

"If you thought the term pig arose in the 1960s, you're in for a surprise. The OED cites an 1811 reference to a "pig" as a Bow Street Runner--the early police force, named after the location of their headquarters, before Sir Robert Peel and the Metropolitan Police Force (see above.) Before that, the term "pig" had been used as early as the mid-1500s to refer to a person who is heartily disliked.

The usage was probably confined to the criminal classes until the 1960s, when it was taken up by protestors. False explanations for the term involve the gas masks worn by the riot police in that era, or the pigs in charge of George Orwell's Animal Farm."

http://www.straightdope.com/columns/read/2209/why-are-the-police-called-cops-pigs-or-the-fuzz

The Straight Dope is often well researched but I see no evidence that the Bow Street Runners had an HQ with a porcine connection.

Sir Robert Peel, the founder of the Metropolitan Police Force who also obliged all UK cities to have a police force (hence police were called Peelers and are still called bobbies), was a keen breeder of pigs. The Tamworth pig originated on his estate and this may well have encouraged the references to his police as pigs.
https://en.wikipedia.org/wiki/Tamworth_(pig)#History

The truth is we that don't really know but, as pigs are known for snuffling into things, just as a policeman's lot often involves having to nose into other people's business, perhaps the comparison was inevitable.