Well it could actually mean a few things.
The worst case scenario is Nate Diaz was cut from the UFC for constantly butting heads with Dana White. Granted, that’s the most bleak scenario. Nate Diaz could have also been pulled from the rankings simply because he’s a difficult employee to work with — still, a pretty horrible reason to unrank a fighter. Of course the most logical reason would be that there’s a glitch on UFC.com that accidentally pulled Nate Diaz from the rankings. Yeah, let’s hope that’s the one. Props to @ekc for the find.