Then barring any drastic carb differences (basically you overcarb your own beers with a long line and now a beer with a more standard carb is breaking out), which is probably not the issue, it's not something to the overall setup (temp stratification, line setup, etc), and is probably something physical between the keg valve to the adaptor.
You're either getting gas mixing, or a leak. I've seen tiny leaks that seal under the pressure of a closed tap, but once the tap is open the pressure drops and it seems like you get some venturi effect drawing air into the line (me theorizing there). Seen it when the wrong o-rings are put on, or too tight (not tight enough it tends to leak beer out). Or, the leak could be what's causing breakout. And once you get a little foam in the line from either, it creates more foam.
Given that this is a retail circulation keg, I'd still get a flashlight and look really closely at the valve on the keg itself. That rubber seal is all that separates the gas flow and the liquid flow, and if that seal is damaged, you push CO2 right into the beer line, and that's your issue. It's a very normal occurance. They damage with time (rotting out and needing period replacement), or, if someone forces an S-type coupler onto a D-type keg valve, it can damage it that way. Or any other form of abuse. The brewery (or whatever keg fleet) might not even be aware of it (although most breweries, responsible ones at least, check those when filling them). If there's any cracking or chipping to that valve seal, that's likely your problem. In which point I'd call up the place you bought the keg, explain the issue, and ask to swap it out for a replacement keg.
If that valve looks good, I'd rebuild the coupler. Pay very close attention to the o-rings- the one that goes on the end of the coupler probe looks very similar to a standard hex nut o-ring, but is actually slightly larger. If it's a brand new coupler that's likely not the problem, but I've seen the wrong ones get put the wrong places and cause problems.
