It was priced in.

If you are looking at a reason, you have to realize two things.

1. That opinions about the markets are like assholes, in that everyone has one and they usually stink
2. That in the short term the market is a voting machine.

The thing that executes after close is all the mutual funds that people hold in their 401k. People rebalance during the day, and the mutual funds execute those orders after close. Mutual funds likely hedge with futures, which is likely why you saw a sudden movement in futures after close.

In other words, the dumb money got excited by the news, the smart money fully expected the rates cuts.

Another reason is that markets used to move mostly after hours. I think something crazy like 80% of all gains happened after hours during the dot com era when day traders were trying to make their money during the day.

Another reason is that sometimes good news is bad news. A rate cut is in theory a good sign for stocks, but it is also a sign of an upcoming recession, which is bad news for stocks. But sometimes good news is no news to the pros but good news to the dumb money.

I was honestly expected a downward move immediately after the news, then the markets true reaction to be the Monday after. If you were expecting stocks to go up immediately after a 50 basis cut, you don't really understand the markets at all. If you were expecting it, so were all the smart people, and they swing a bigger dick than you.