If I had to guess, you have a hole in your dip tube up near the top and it's pulling a stream of co2 into the output on the way past.
If you pull the dip tube out and seal one end with a finger, blowing in the other should let you know if it's the dip tube. If you can pressurize it and don't have to replace the air, you're probably good, if it hisses, it's time for a new dip tube, or an adventure with silver solder, if you roll that way. Sanitize it before putting it back in the keg.
I had a whole batch of cider I served all foamy and I assumed it was because cider is weird but when I took it apart, I found the hole in the dip tube.