It was as though a large buyer and seller magically approached the market at the same time and met each others demands to the tune of 800 BTC. Bid/Ask depth remained constant throughout.
I have also seen suspicious trades occur way off the mean like your chart shows a few price spikes, yet the volume in the order book seems unchanged. It seems the only way for this to happen is for a coordinated buy/sell with someone in control of both accounts. I have seen some exchanges (btc-e) report trades out of order (based on time stamp) and sometimes several minutes later. I have not tried to track their order book vs. trades to see what gets filled and if it looks real. There could also be a significant delay in the book depth as well.
The thing that is most suspicious about these events is the price seems unaffected. If the book depth was truly eaten into then the price should rise at some point even if it is delayed.