There are several reasons:
1. After the end of WWII, the commie government decided to rebuild Warsaw, that inflicted a serious damage during the Warsaw Uprising, and recreate it's former beauty. It was suppose to be a testimony of the People Republic of Poland’s power, thus Warsaw was treated more special than other major polish cities. People didn't necessarily like that.
2. Legia Warszawa - Warsaw’s football club. It was an Army football club. The commie government wanted it to be the best Polish club. A lot of good players from other Polish teams were specially being drafted to Army and were given a choice, whether they wanted to spend 2 years in army barracks or on the football pitch playing for Legia. Many people didn’t like that either.
3. Varsovians also are rumored to consider themselves better that the rest of Poland’s population, just because they are from "stolyca"(capital city).
We could probably find more points if we would look deeper into the past. Times changed, but the animosity remained. :)
Quoting: Grzegorz_, Post #11
Don't tell me that you are from Szczecin...
I am, but not originally. It's where I used to live for some time