Is it a matter of multiple players not coming close to getting the most out of their ability? Or multiple players having career years last year who are now playing at their true talent level?
Those are not really the best options.
Sometimes, guys at the same talent level just don't have the same season one year to the next. It's an unpredictible, non- linear game in the particulars. Actually, that reality is totally expected in baseball---we just don't know in advance the identity or degree. Sometimes, happens in bunches. Part of the game.
Expectations don't necessarily even-out in a given season between expected performance level, under-performance level, and over-performance level. Often, it's just random.
Don't think anyone really understands the game if don't appreciate the randomness of how things can play out. Sure, a guy may play below expectations for a time because of family issues, health issues, other stuff issues---all part of the game.
So, there are no grand explanations most of the time. Yeah, you can lose your two top hitters and two top pitchers all season for injuries--you have a grand explanation! But, more typically, not that neat.
And, it's still 85 games, not a full season.