I think we underestimate how much real life can impact the performance of a professional athlete. Being traded, sick child, sick wife, sick parent, financial troubles, etc.
Think about how terrible Jean Segura was in 2014-2015, which is when his child was sick and then passed away. A lot of people were ready to write him off but he bounced back and has put together a very nice career (27.9 bWAR).
These elite athletes are at the very edge of high performance. I can see where it doesn't take much to throw things off.