So, I've had this happen to me twice too. My bike's a GS500, but things probably aren't that different. AFAIK it can only really be either a bad petcock or (more likely) a stuck float or something that prevents a float needle from closing properly (e.g a stuck float or debris between needle and it's seat). This makes the carbs overflow, causing gas to pour into the bottom of the airbox and out the airbox drain hose. Path might not be exactly the same on your bike, but in any case there's too much for the carbs to hold and by some path it gets to the hose.
When this has happened, I've been able to fix it (in the short term) by doing the following:
1) Turn off tank mounted petcock (maybe your bike doesn't have this, dunno, either way, turn off whatever you need to turn off to stop fuel from getting to the carbs). If you don't do this, when you do step 2 gas will just keep coming since you haven't turned off the supply.
2) Drain carburetor bowls by loosening drain screws. When this happened to me out on a ride, I got a paper cup from the fast food place whose parking lot I was in and cut it down until it was about 2 inches high so I could fit it in the right spot to catch the gas coming out of the carb bowl's drain tube.
3) For good measure, give the carb bowls a few solid taps (I wouldn't quite say 'whacks') with the handle of your screwdriver.
Both times this has caused the problem to go away for at least a week.
In my case, I'm pretty sure what's causing the blockage is rust particles, as there's some rust visible in the bottom corners of my tank. I'm going to clean this out and reseal the inside of the tank over the winter and then clean the carbs again. Season's not over yet though, so for now I carry the tools I need to fix this on the side of the road if it happens. Fingers crossed that it won't happen a third time.