There is a coolant fill level on the bottle.
As the vehicle warms up the coolant expands and it's forcing it out of the system. It should be a few inches from the top. Check the bottle (expansion tank) for a line that says full (hot) and keep it at that level.
Overfilling it will do exactly what you describe.
The two bumps that are visible in the photo show the high and low levels. It IS actually called an expansion tank but due to the massacre of the English language and slang it has fallen by the wayside except to a very select few.