It's SE telling you that you've played enough, and they want you to do something else.

Joking aside, it is an "innovative feature" that is under some bit of controversial discussion. It's called the "fatigue" or "surplus" system. It is intended to stop players from playing one job too long. SE wants for players to level many jobs at the same time, instead of focusing on just one or two, so that they won't progress too quickly through the game. Ultimately, it's just a big pain in the arse. But hey, they'll probably(hopefully) change it soon.