I have noticed that too this summer. Here is a paragraph about sycamore bark shedding:

Sycamore trees are known for their distinctive peeling bark, which is a normal part of their growth process. This shedding is result of the tree's rapid growth and the rigidity of its outer bark, which cannot expand with the tree's trunk. The bark peels off in irregular patches, revealing lighter-colored inner bark, giving the tree a mottled appearance. This shedding is not harmful to the tree and is considered an ornamental characteristic.

We have had a wet summer so far so sycamores may be growing a bit faster than they normally do. 

Thanks for the quick response. We have another sycamore, same age and height that isn’t shedding nearly as much and it doesn’t have as many of the very light patches. In 20 years we’ve not seen this much shedding- that’s why the concern. 

I do not believe this is anything to be concerned about. The sycamore trunk structure (with limited bark) looks normal and the amount of shedding, while unusual, is something we have been seeing this year. I think it is due to rapid growth. The fact that it is occurring in one tree and not the other is also normal. There is normal variation between specimens even within the same species.