I have a vented cap and well. I checked it over to make sure nothing to make it looked fine. This issue just started recently and even happened ( to a much lesser extent) at my buddies shop when the car was sitting and the engine cold.

When it happened to me at a car show last month it dripped out steadily from both side of the tank like it had a hole in the tank for a solid hour. I thought the seam had gone bad but that not the care as on the life you can see it coming out of the smaller curved vent tube.