Yeah, not sure really! I was thinking maybe pressure in the tank or fuel line forced it past the seats or petcock? My tank vents to a catch bottle. Maybe the line is pinched? I do run a fuel filter and between that and the line to the tank there would still be gravity pressure on any fuel in the line.

I would say the amount of fuel that shoots out the pipes is about the amount in the fuel line and carb bowls.

I have had the tank off a thousand times and I know the petcock works. Each position has a nice snap to it when you hit the detent. I know I had it in the off position and its done this twice now. I am certainly going to test it again. It's been a few months since I had the tank off. Maybe the seal in the petcock has failed.

It's always when its on the side stand too, never when it's on the rear stand. I am going to pull the carbs and check them out regardless, petcock too. Funny thing is it never pushes fuel out the bowl vents which I think would happen first before it pushes its way past the seats.