We know this: starting pitchers are throwing harder than ever. Because of this, and the limitations of the human body, it’s harder than ever to throw a lot of pitches.
This trend goes long before 1974. Long gone are the days where Cy Young started both games of a doubleheader.
“Bullpenning” is a term used to describe relievers taking the place of starters. Bullpen arms will often pitch two innings or fewer across the board, with around six to ten pitchers completing a nine-inning game.
The reason behind a bullpen game varies: it can be to work around a starting pitcher injury, to allow the rotation an extra day or rest, or simply to give the bullpen some work that starters may have taken away in the days leading up.