A lesson learned:
On my setup, the pressure equalizes between all the kegs. So the instant you plug in your uncarbonated keg to the system, pressure will come from every keg and the tank. Sooo, if you have any kegs filled really high, up beyond the gas line dip tube, you'll suck beer back into the gas lines.
Long story short, make sure you don't fill your kegs too high if you're going to be tinkering with pressures, disconnecting, etc.