The peeling bark can be a part of the natural aging process of the Cottonwood Tree.
They are not terribly long living trees. The bark sheds and leaves the white underwood exposed and the tree will decline.
They tree can also be suffering from disease or even Winter damage.
If you have several trees dropping bark, examine the tree for any other clues or signs of disease or insects.
Are all the trees of the same size and age?
A certified Arborist can examine the trees and determine the exact cause or if any treatment may be necessary.