What would make a hose pop off .

It is a small hose by the top but I can't see where it would be connected to. My car started to over heat as I was driving so I pulled over and saw that hose with what I thought to be some kind of fluid I think radiator fluid. Then I let it sit a minute and was able to start it up it didn't jump to hot but I didn't drive it. It is making a clunking rattling noise .

vacuum hose maybe?with car running,can you hear hissing sound anywhere(sucking air sound)?look on engine for small nipple or connector that hose may connect to.