G Coupler "Popping" Off With Force

[tjmile1]

I have a Micromatic "G" coupler I use with Boddingtons (Nitro) and I've noticed that even though the handle is at it's uppermost position it "pops" off with considerable force and sprays a little beer when unscrewing it from the keg. What am I missing about this type of coupler? Most I can just shut off the gas, unlock the handle, and turn it to remove. This "G" coupler shouldn't do that, right?

This coupler's handle is just straight up to untap and straight down to tap, so I don't get it.