One possible explanation for early bidding that hasn't been mentioned is when a buyer (who is not relying on a fancy multiple auction sniping program) is interested in several similar items on auction at the same time, but only wants to win one of them. Thus, the buyer may "test the waters" of the most attractive item first, even though that auction is not the first one to end.
From a buyer's point of view (most of the time), Dutch-style sealed bids would be a nice alternative to the Ebay model. That ain't happenin'.