[shmoogy]

It was 4 shares: https://www.nasdaq.com/market-activity/stocks/brk.a/after-ho...

Likely one person that placed a market order

[skzv]

I don't think you can place market orders after hours.

It could have been a deliberate limit order to avoid a margin call by raising the mark-to-market value of an account.

[cbhl]

That sounds like an expensive mistake -- BRK.B doesn't show anywhere near the same price movements.

One does not simply place a market order during after hours trading...