In the Everybody Loves Raymond series, why does Frank (the Grandfather) always opens the zip of his pants when he sits down in chair?
People sometimes undo their pants when the are bloated after a meal. The ever-uncouth Frank does this routinely, in front of others, reminding us that he really doesn't care what anyone thinks about him. This was a running gag that appears to have been originated by the creators of the show.
In a book about the creation of the series, You're Lucky You're Funny: How Life Becomes a Sitcom, Phil Rosenthal writes about the casting of Frank Barone. They were looking for a "nonethnic ethnic" to contrast with the more ethnic Italian/Jewish Ray Romano and Brad Garrett. When Peter Boyle arrived for the audition he was furious because he had had trouble finding the place.