The explanation I can think of for these middle-of-spread trades is a buyer and a seller (perhaps the same person) agreeing to post matching orders at the same instant at that price.  Such "rendez-vous" trades could be a way to launder bitcoins and/or money in a way that would be difficult for the police to trace, even if they had access to the deposit and withdrawal records of all the exchange's clients (which the exchanges presumably provide).