First I've seen since back at the end of May, but its very possible it happened and I missed it...especially when I was on vacation down east and not checking as closely. 

As for the rationale, investopedia notes that buyers will sometimes use a MOC order to close out the unfilled portion of a limit order.  That is to say someone might place an order for 2,500 shares at $3.00 but only get 1,900 filled so they have a MOC order placed for the final 600 shares.