Although I can't think of other examples right now, I noticed while re-watching The Office that the characters started cussing more and more as the series progressed.
For the first few seasons the language was more so than not PG rated, but, around season 4 and 5, and definitely in season 6 (when Michael drives into a pond) they start cussing more frequently and more extremely (even dropping the f-bomb). And then, once Michael leaves, they REALLY start cussing (with Angela even saying f***!).
I really do feel like other TV series have also exhibited this behavior, and if I think of them I'll make an edit.
So, why is this the case?