Probably condensation. Since it sounds like you don't have the vent fan from the microwave attached yet a lot of the air from the kitchen was probably just drifting up there and the cold air from the outside and in the attic made it condense on the duct.
Make sure you wrap insulation around that vent duct in the attic and you might think about getting an exhaust fitting that has a damper. Some string around the insulation will help keep in on the duct or look for some insulation specifically made for goind around a 4" duct.
Once you get the microwave vent fan on that will keep most air from drifting up and the roof damper will keep some of the cold air out of the duct and when the fan is on the insulation will help keep the moisture from condensing until it gets outside.