If you were watching the walls you would have seen what happened here. Sudden very strong downward pressure being put on Stamp with unusually tall sell walls going up bang in the middle of the spread every minute and then disappearing when the price moved down a little. Then bang up again. For 45 minutes.
Isn't that just someone trying to use Bitstamps silly alternative for a market order, their "Instant order". They describe it as:
Quote
When you place an instant order, we add an order at the current bid or ask price (depending on the type of the order). If there is not enough bitcoins available at that price, your order will remain open until someone sells or buys additional bitcoins at that price. Our trading engine will then reposition your order every 10 minutes to the current order book offer.
This sounds like what you're describing: A wall right at the current price, that sits there for a bit and then disappears and moves down with the price.
