This was a bonus in the Midwest program where members receive a 5,000 mile bonus when they hit 30,000 flown miles in a year, then another 5,000 bonus miles again at 40,000 and at 50,000.
I have not heard either way about that being brought over to the Frontier program. I think it was a relatively new addition to the MM program and am not sure if other carriers offer something similar or not.