I'm not an art historian but that doesn't stop me having theories. When we think of light infantry caps of the age of reason we tend to think of them as being primarily practical but I think there's another reason for their appearance in the American Revolution and that is the classical revival. So many of the distinctive caps of light infantry and dragoons seem to be based on ideas of Greek and Roman designs it could be an echo of what was going on in architecture. The legion as a concept was pretty popular in the Revwar also echoing the classical models of military excellence. This image from Warley Camp in 1770s shows some of the fanciful headgear worn by British light troops - a long way from an item of practicality.