I believe what is going on here is that a very large block trade is occurring off exchanged and being reported the the consolidated tape in a delayed manner. For example, a stock is doing it's normal volume and normal liquidity being displayed on the book. Two parties agree on a block trade for an amount of stock that is much larger than the average daily volume at the price where the stock is currently trading. The parties then report the trade to the consolidated tape, possibly hours after the trade takes place and thus it shows up on your chart. My software allows me to see charts with just data from New York which would probably eliminate that print (although ECN volume and price action is important so I'm not recommending only looking at New York). I think this is what is going on but someone may know otherwise.