Highly unlikely that this would happen with two kegs in a row but the seal on top of the keg can cause this. If you are using a
"D" style valvesystem, the seal on top can become damaged where the pressure flow routes into the beer flow instead of going into the keg as it was intended. Sometimes, by moving the couple slightly back and forth this will overcome the seal damage and pour fine.
Another issue may be the check ball in the coupler probe may be stuck. Keep clean and it should be fine.
Scott Zuhse, Instructor
Micro Matic Dispense Institute