1994 Mitsubishi Mirage Gas pouring from evaporative emissio

I recently replaced the entire fuel pump assembly and the fuel level sending unit on my 1994 Mitsubishi Mirage LS Coupe. (It has the 1.8 L engine with an automatic.) The car ran fine on a couple of short trips (1.5 miles each). At that time the gas tank was nearly empty. I then drove it about three miles to a service station and put in about 5 gallon of gas. I then drove it about 2 miles when the check engine light came on. When I inspected the engine I found gas pouring (not just dripping) out of the vent hole in the bottom of the evaporative emissions cannister. I loosened the gas cap to relieve pressure from the system and eventually the leakage stopped but it did not stop right away. What would cause gas to pour out of the bottom of the cannister?