Any time you put an additive into your gas tank you should put the additive in first, then fill the tank with gas. This causes the gas to mix/dilute the additive so it doesn't have a chance to get sucked up nearly full strength into the fuel system. There's no way (that I can see) that the additive can leak out of the fuel tank or filler neck without gas leaking out also. You must have filled it up with gas, then the fuel treatment didn't have anywhere to go but overflow down the filler neck when you weren't paying attention.
It's either that, or like
@Dirtman said; magical fairies. They can be worse than gremlins.