Unusual high volume after market close price action explanation [with charting to illustrate]

External question — Personal Finance Stack Exchange Author: Kaguya Ōtsutsuki Original post: https://money.stackexchange.com/questions/133932 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. The volume of the day including pre and post: ~122M The volume in that 5 sec encompassed 17/122M= 14% of the trading volume of the day. Can anyone try to explain this phenomenon? Some possibilities I think of: Day traders taking profits Hedge funds adjusting positions but don't want to plummet the price Unheard of ETFs emulating the closing price None of these fully explain it, because why would they intentionally wait till the market close to start/close positions when they might run into the risk of lower liquidity, higher price volatility? Updated: I checked with the tape and it was due to one single transaction (the time is in PT, NOT ET):

External answer — Personal Finance Stack Exchange Author: Bob Baerker Original post: https://money.stackexchange.com/a/133934 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. You have a huge volume spike at 4 PM. Seven minutes later you have a bizarre candle where share price drops almost $1.00 and is completely recovered within a minute. So the first possibility is that it's just bad data. In lieu of bad data, a legitimate reason for such a large volume change could be a cross trade because a broker executed matching buy and a sell orders for the same security across client accounts and then reported them to the exchange at 4 PM. This is allowed as long as the cross trade price corresponds to market price at the time of the trade.