Almost 1/5 of today's volume was in that one minute this morning when 23,000 shares traded. The more I think about it, the more I think it might have been someone having to cover a short because of a margin call. Remember that it happened when the market was heading down big. If someone had a large long position in some stock or ETF that got hammered, they might have needed to close the short. Or possibly, if they were using a brokerage like Interactive Brokers, the broker did it to them. (IB will close your position quickly, and you have no recourse because they can say "we thought it posed a risk to you." Read your brokerage agreement if you have an account with them.)

The reason I think it might have been short covering is: if you were trying to actually buy 23,000 shares of stock, you wouldn't do it all within 1 minute when the volume was really low. You'd space it out with multiple trades of a few thousand shares at a time.