Strange. Several million shares of B shares sold within minutes of close...

Stuff you see as "after close" trades often aren't. They're often block trades that are reported after close, not necessarily trades that actually took place (initiated+filled) after close.

Since there is some obsession about option expiry dates, note that typically brokers will automatically assign expiring options that are in the money. If those are calls, they need to buy large amounts of stock for you at "market on close" (MOC) price. I would have expected those to get reported as volume of the closing auction volume, i.e., end of trading but technically within trading hours, rather than after hours. But (a) I don't really know, and (b) wild speculation it could be both factors: brokers settling some MOC trades among themselves and then reporting them?

Other factors for the obsessed:
Stocks with large numbers of deep-in-the-money options outstanding tend to have price weakness at expiry dates as assigned stock gets sold, followed by a rebound.
Stocks with large numbers of near-the-money options outstanding will tend to close very close to a heavily used strike price, on high volume and volatility towards the end.

I'm not sure any of that matters, of course. One might speculate that Berkshire might outperform the market on Monday if the "rebound" effect above happens. Buy a share before market open : )

Jim