A poorly constructed chimney will do this chronically, or a chimney that hasn't been cleaned for a while, or with a birds nest of broken top may start to smoke. Also, strong wind can make an otherwise sound chimney smoke - during gales we would get the ocassional blow down in my parents house. The Admiral notes it only happens when the wind blows a certain way if I recall correctly. There are all sorts of fixes from a good cleaning to chimney pot caps to help with the problem.