The Phoenician city of Tyre is a bit of a melting pot. It is a port city, and most of its inhabitants are connected, one way or another, with the sea and the trade that it brings. Located close to Syria, many of the Tyrians are a mix of Phoenician and Syrian, and for the most part, they follow Gentile religions – worshipping the Greek, Roman, and Syrian gods. Because of this, they are considered by the Jews to be “unclean” – just as “unclean” as scavenger animals are. There are many scavenger animals that live in Judea, pigs, shellfish, rats, and dogs are some examples. But the Jewish people stay as far away from them as they can, so that they will not defile themselves by having anything at all to do with them. No good Jew keeps any of these kinds of animals as pets. Over time, Gentiles have come to be referred to as dogs, including those Gentiles who live in Tyre, because of their “uncleanness” in following other gods. They all know it, and know why. And it does not do much to improve relations between the Jews and the Gentiles.
http://www.bible-history.com/geography/ancient-israel/rivers.html, http://morechrist.blogspot.com/2013/03/jesus-acts-like-racist.html, Paraphrase on Mark, By Desiderius Erasmus, Erika Rummel, page 93