http://www.kia-forums.com/3g-2011-2016-sportage/182722-recirculation-button.html
It turns off whenever you use AC or Defrost because the car draws in fresh air. You can't override this while defrosting. It is how the AC system works, and the defroster uses your AC system to function. If it didn't turn off the recirculate, it would keep fogging up your windows worse with the moisture that you and your passengers exhale when you breathe.
It turns off automatically when you start your car to allow fresh air into the vehicle again in anticipation of the need to defrost the windows, or run the AC in the summer.
Both are automatic features of all cars that have a recirculating option.