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

Question

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):

Bob Baerker · Accepted Answer

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.

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

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