I'm willing to bet it did gush out before...probably every time you topped it off and then went for a drive. You just never had anyone next to you at the light tell you about it happening before, and it finished burping out while you were driving down the road so you didn't know it happened.

Cooling systems shouldn't need to have coolant added...ever. If they do get low, it is because they have a coolant leak somewhere that needs repair, or because they've been overfilled each time...followed by burping out all over the pavement when you're not looking.