Chuck Cunningham Syndrome is the tragic loss of a TV character who just wasn't interesting enough to keep around on the show.
These people might have been around for years, or maybe only a couple episodes, but one day they vanish from the face of the earth. Never to be seen or heard from again. Why does this happen? Do the television Powers That Be think that we're just not going to notice one missing character? Come on. We're fans. We would notice if one of the characters was eating a sandwich with peanut butter and jelly when four seasons earlier that very same character had a throwaway line about a peanut allergy. These are the kinds of things that make the world of Tumblr go 'round.
So why do they choose to send these characters off into oblivion? Well, there's a few reasons.
Maybe the character isn't going over as well as it should. Maybe they realized that having five kids in the family is too complicated and expensive, so they decide to downsize. It is a rough economy, after all. Maybe they don't know how to write for them anymore, maybe they think that their story line has played out and they don't have anything to contribute, but regardless, there's no excuse for the complete lack of thought in writing these characters off the show.