FFT 9020/9021 (TPA-JAX)

I can’t figure this one out…Frontier served JAX from DEN for a while but ended service a few months ago. Lately, I’ve seen some late night flights hopping up here from TPA, only to turn around and head back an hour later. I know 9xxx flights are typically designated as charter or positioning flights, but this just seems really random. Maybe they still had some equipment here in JAX that they’ve been loading up and taking back to DEN from TPA in the morning? Anyone have any other thoughts/input?