Pulled the data to look into this and... my goodness, it is a flurry of intermarket sweeps: 552 ISOs in a span of 7-8 seconds. This is inexcusable: There is no way that much volume needed to be crammed into the market in the few seconds between 14:12:05 (when sweeping begins) and 14:12:12 (when sweeping ends). I cannot see any way to argue that this was "best execution." This was terrible execution and would be reason alone to fire a trader on every desk I have sat on. You might even argue failure to supervise because a good smart order router should warn you that doing something like this is not sensible.
What I previously said:
This could happen if somebody was stupid, set flags to indicate they did not want to get routed to the best orders because they wanted to trade on a certain exchange, and got terrible pricing as a result. This could also happen if that person sent a very aggressive IOC order.
However, I doubt that is the case: Most smart order routers would not do this and it would open the door to accusations of manipulation -- since you could not argue that a price so far from the inside market could be justified by rebates or payment-for-flow. While the S&P 500 was down at most 1.6% (open-low), that still would not explain this CTAS print at 3.7% below the open.
The most likely explanation is another reason you will see trades print far outside the spread: if they are average price (VWAP) or prior-reference-price trade prints. (These are often used for block trades.) This is one of the exceptions to order protection, as per Rule 611(b)7. These prints essentially record the aggregation of trades done before that time, across multiple venues, and possibly including off-exchange block trades. Given the large size, I suspect this is what happened. There is no way you could have participated in this trade -- since it was likely arranged over a period of time long before that print.
A good broker would likely send ISOs to try to fill orders sitting out in the market to reduce the price impact; however, given the (sometimes) complications of negotiating these trades, that is often not required (and may not even be practical or possible).