I am not well versed in this area but tangentially from studying other aspects of Roman culture I gather that Hadrian was most likely responsible for the rise in Philhellenism and the more general wearing of beards. I and I am terribly sorry if I am incorrect but I think Dio Chrysostom is the one who mentions Hadrian as making the beard fashionable with Romans. I would assume members of court would in a sycophantic fashion be keen to copy fashions of the Emperor as to better improve chances of patronage. found these books fairly good studies of Hadrian Anthony Everitt, Hadrian and the triumph of Rome 2009 Hadrian's empire : when Rome ruled the world 2006