What causes airfare prices to be incredibly cheap at times?

For example : On atleast 3 different occasions, I have stumbled upon airfare from Milw, WI to Phx , Az For only $40 one way. It is usually Frontier airlines. which offers these deals. But last time it was American Airlines. What causes flight prices to be this low ? When can I get lucky and find something this... show more For example : On atleast 3 different occasions, I have stumbled upon airfare from Milw, WI to Phx , Az For only $40 one way.

It is usually Frontier airlines. which offers these deals. But last time it was American Airlines.

What causes flight prices to be this low ? When can I get lucky and find something this cheap again?
8 answers 8