These engines are notorious for getting pockets of air trapped in the cooling loop. In many cases, bleeder valves are placed on the manifold (the highest point on the cooling system) to evacuate the air.
Chances are there was some air trapped in your system and it burped, popping your reservoir cap and spilling coolant.
Depending on the type of manifold you have on your engine, there is most likely a brass plug somewhere along the top surface. When your engine is cool, remove the plug and check to see if you have coolant. If you don't, there is most likely air trapped there. Leave the plug open and begin filling your expansion tank. Once the coolant level reaches the top of the plug (not the expansion tank), stop filling.
You want to make sure there is no air in your cooling loop. These engines do evacuate the air rather well, but it often results in a nasty burp similar to what you experienced.
Also, give your engine a cold start with the tank to your expansion cap open. If you see bubbles coming out if it, let us know. The problem then becomes more serious and lies outside of trapped air.