Well, it was quite a while ago. The "Ridings" are a corruption of the Norse or Anglo-Saxon word Thridding, meaning "third". I presume that the whole county would have been too big an area to administer properly with the means of communication in those days, so it was split into smaller parts where the peasants could be squeezed more efficiently.
Yes thank goodness they gave us back our East Riding, my Brother in Law continued to address is letters thus as a protest. Quite right a riding is a third and therefore there could never be a South Riding as some have us believe its not mathmatically possible, so there.! A Yorkshire Lass.
The split occurred with the 1974 local goverment revision. The three ridings of Yorkshire were split between the five new counties of North, South and West Yorkshire, Humberside and Cleveland.