In addition, he is performing invalid sacraments, and leading people to believe otherwise.
Zigrang's belligerence, his year of indoctrination in SSPX propaganda, and basic lack of courtesy to even respond to Fiorenza's letter indicates he's decided to join the schism.
"his year of indoctrination in SSPX propaganda"
There are over 300 Diocesan priests taking instruction in Latin and Canon Law and the Liturgy from the SSPX right now Deacon. Get over it. The revolt against the sodomite cabal is in full force.
He has the right to say the Tridentine Mass. If his bishop denies him this right, the bishop is the disobedient one, not the priest, since Quo Primum has never been abrogated. Fr. Z is on very firm legal ground.