Reversed flow in faces on pressure outlets
 

I am simulating a 1-meter height 0.15-meter diameter of a cylindrical bubble column reactor. where the liquid is the primary phase and the air is the secondary phase. After initialization air is patched from 0.3 to 1 meter of column remaining water is filled initially from 0 to 0.3 meters of the column. While running the simulation, a message is shown that reversed flow in faces on pressure outlet in each iteration. Why is showing? please explain it?