Gasoline leaking from overflow spigot

I've had my boat winterized for about 1 month now and yesterday gas starting spewing from the overflow. It's been untouched in my garage so I'm not sure what's causing this.

I've been winterizing my boat now for 7 seasons and never had this happen. I always fill up the tank, but never top it off.

Any opinions on why this is happening?

Ok, these two threads touch on the issue. One solution is to add extra vent line and do a high loop:
Thead 1
Thread 2

Here's a product from Skidim that says it helps with the issue:
Ski Dim Fuel Vent

If you give the year and model of the boat as well as the engine, you'll get some better and more specific advice. The solution could vary depending on whether you're carb'd or fuel injected, size, shape and setup of your tank and fuel lines etc.

Most all fluids and gases(not refering to gasoline) have some rate of expantion. Most station's fuel pumps are designed to kick of prior to spilling out, howver every pump is diffrent. My guess is that that little or no space was left allowed for the fluid to expand. Temp changes play a big part. Is the boat covered? Indoors? Is the aft in the sun or near a source of heat? Is the wall near the aft on the south / west side? Weather changes?

Try cracking or opening the fill cap a little and see if it helps.