I have family members who were impacted by this so a little research turned up that passengers were claiming that the pilots all walked off the job. My own relative said that after she sat around waiting for 7 hours looking at the plane but not being allowed to board the crew got off - no pilots among them - and the flight was cancelled. Spirit insists there is no issue with pilots but I have to wonder.
Spirit airlines: okay, all pilots must get the jab or get fired!
Pilots: see ya!