This all started form a segment called "Human Extinction Watch" where he focuses on how Phillies manager Ryne Sandberg got food poisoning from a burger at Shake Shack. Shake Shack is a big name in New York city and it is hilarious to watch Stewart go on his rants when people bash things about New York (i.e. the time he went off on Chicago style pizza). This doesn't really involve Pittsburgh at all, but I think we can agree this is pretty awesome.
It does actually kind of reference Pittsburgh when Stewart starts the rant with this:
"I'll tell you what's happening here. A great New York restaurant is being slandered by some jagoff from Philadelphia."
The portion that I am talking about starts at the 4:10 mark (the whole segment is actually really good).