Maybe your bladder was too full from the problem you describe in your other post, and the fluid had to come out somewhere. 8-)
Sorry, but it just struck me as funny when I thought about how you posted about two unrelated problems, but both involved the expusion (or non-expulsion) of fluids.
Seriously, though, I have had a similar issue with my ears. Water has suddenly come out in volume several hours after surfacing. It is a little more obvious how the water would get in an ear than a nose, but the point is that it can stay in there quite a while before draining. In combination with the other theory on drainage from ear to sinus and out nose, this could be what is happening to you. Another way could be that the nose pocket of your mask is flooding a little and the water makes its way up the nose.
I find the water in your breathing gas unlikely as the cause, since mouth breathing does not go through the sinuses, nor is it likely that enough humidity could build up to cause that much water to accumulate.