I have a steam system in a 2 floor house with a basement. It has a dry return. When the heat goes on all the radiators work fine, but one of the radiators on the second floor for some reason fills up with water and starts leaking from some of the joints. While the heat was on I took off the air valve and water started gushing out. I can't figure out for the life of me how water is filling up in the radiator that's on the second floor!! Somebody please give me some ideas so I can fix this myself. I have intermediate knowledge of steam systems.

The shutoff valve is also new so I know it's not clogged, the water should be returning and the radiator leans slightly toward the shutoff valve for the water to return.

Congratulations to the person who knows what the problem is and walks me through this one. I have to say I'm a little skeptical. I've already asked half a dozen people and no one could explain this phonomona.