As long as several of you are voicing your speculation, I'll add another possibility (much more of a long shot).
Frontier's announcement of an impending IPO, followed with the intended purchase of a new headquarters building in Denver, prompted a major player (I'm speculating Southwest or United) to make a respectable offer to outright purchase Frontier before these events happened.
The potential buyer has asked Frontier to suspend hiring (to more favor the potential buyer's current seniority list employees in a future SLI) until the offer/sale can be evaluated, and/or completed.