Given the size of the trade, I think it was a negotiated block from earlier that the broker put through just before the end of the day. I see (through my rose-colored glasses, perhaps) it all bouncing back sharply at the beginning of the week.
Seen this a few times before. In fact 3 of my other stocks were doing well into close and all three dropped sharply just like this one in the last 30 minutes of trading. Predicting a rebound Monday unless something is seriously wrong. One thing for sure, a lot of stop losses were taken out and a lot of orders on the bid were filled. Hopefully everyone is happy with that.
I believe this major sell-off at the close was related to the quadruple witching and that an initial spike down on account of it cascaded some weak hands. If a bunch of sell stops set in the high .30s were triggered, it may have forced others to get out based on the implied "somebody must know something" action, as well as other and further sell stops.