He was being floated as a cap casualty; maybe he was approached to redo his contract and balked... or maybe he decided he doesn't like Brian Flores or his vision of the defense next year.
Z is a little bit of an oddball. Who knows with him.
EDIT: current spin is it's about money.
https://www.msn.com/en-us/sports/nfl...ay/ar-AA18r6gt10 minutes after Smith's tweet, NFL Network's Ian Rapoport reported that while Smith had requested his release, likely in pursuit of a more lucrative deal, the Vikings are actually not interested in cutting him loose.