There was a bitty more to it than that. Yes, Southwest starting DEN-DAY caused problems, but Frontier has to have an aircraft at CVG for the Apple flights to CUN and PUJ. They had been doing this by repo, but that was expensive and it's obviously cheaper to shift the DAY flights to CVG.

Same thing happened at CAK and CLE. The A320 at CLE not only serves CLE-CUN/PUJ but also PHL-CUN/PUJ and makes a (short) repo to PIT for those CUN/PUJ flights.

They had been getting the aircraft to CLE with a repo flight from DEN, but was expensive and Southwest at CAK became the kicker for Frontier going to CLE.

CLE proved to be a smart move and they're hoping the same will be true for CVG. Early indications are good - the Cincinnati press reported the first few CVG-DEN flights as sold out.