Some say that it was just a fad that started with Napoleon, but I think it predates him. I think it was considered a stately pose--perhaps the "shocker" of its day? (Or artists didn't know how to paint hands.)
This article probably explains it all in excruciating detail:
"Re-Dressing Classical Statuary: The Eighteenth-Century 'Hand-in-Waistcoat' Portrait." by Arline Miller. Art Bulletin (College Art Association of America), Vol. 77, No.2, March 1995, p.45-64.