Hey Chris H:
I made inquiries. I believe your sycamore had a bad case of anthracnose, caused by Apiognomonia veneta (the g is silent). The major leaf fall is not a major concern, unless it recurs several years in succession. That could deplete its energy reserves. But it is unlikely this will happen. Control is mainly by raking up the leaves and removing them from premises. (My guess is that putting them in a closed compost system would work fine, too.) You could spray fungicide in the Spring, but only if you have a bunch of money and are trying to figure out what to do with it. The dead limbs are probably unrelated to this disease, and might just be normal senescence (age).
I think the most important thing is to memorize the name of the pathogen. Maybe we should rename Dale's Double Stump as Apiognomonia veneta. After all, that could be what killed the trees that gave us the double stump. All in favor of this new name for double stump, signify by ... oh, never mind!
Chris S.

Chris Schardl