He missed most of last year with a medical issue I believe, maybe he's had enough.
Edit:
Hernandez worked only 10 games last season because of a back injury, but he missed 161 calls, according to Umpire Auditor.
Hernandez was never considered MLB’s worst umpire, according to MLB’s statistical studies and reports, but in the court of public opinion, there wasn’t anyone who missed more calls as a home-plate umpire.