About that 10 kBTC @ 600 offer on Bitstamp: the round numbers suggest someone who bought those BTC when they were cheap, and decided to cash them out at about the current market price; but did not mind if it took a day or two, and did not care much about getting the best possible price.
However other traders had blocked the way with smaller offers at slightly lower prices. So he lowered the price twice by a few cents, just to see some action and make sure that his offer was "working". When he saw that people had started to buy into his offer, he left it there and went to bed.
Does this make sense?