I've had the very same condition on a couple of my daily drivers. Some of them had other motor issues at the time. Like head gasket issues. The coolant will start to rise and fall in the rad while warming and after some arm up time the coolant level would eventually rise high enough to bubble out of the neck. Seemed fine with the cap on. Wish I could tell you why. I don't know myself but I have to wonder if it isn't positive pressure being exerted on the coolant system by a leaking gasket.