I think I know the answer to this, but I'd like to see an answer listing more conclusively the different reasons why this happens. Some causes I've seen listed:
- High pressure plus shaking can cause beer to get into the CO2 coupler
- Dual CO2 regulator hooked up to two kegs, one has higher pressure than the other due to switching gas on and off and regulating at different pressures, pushing beer out when both are opened
This just happened to me and I didn't realize my dual regulator didn't have check valves in the shutoff valves. Luckily beer only got as far as the valves, but not all the way up into the regulator, but I just ordered some new valves with check valves and will be swapping them out when they get here.