Wow. Twice in 5 years. Maybe the 1 and done thing isn't all that great after all.
One-and-dones have been a thing for over two decades at this point. That alone isn't why you're seeing the increased volatility here. What's way more important is that now, in ascending order:
1) There are much more legitimate avenues for top tier talent to reach the NBA than the NCAAs. In the last few years, we've seen many top high school prospects opt to play abroad in foreign pro leagues, or go spend a year in the G-League.
2) The college game itself has transformed significantly in the past half decade or so. College players used to be bad at shooting 3s and take low volume at them traditionally. We've now hit the point though, where everyone who is 18 today, grew up watching and idolizing Stephen Curry or Damian Lillard, and everyone shoots a TON of threes now. And just like in the pros, if you can shoot 3s well, you're never out of a game. A 10 point deficit used to be insurmountable late in a game; now you just have to hit a couple 3s in a row and you're right back in it.
3) The transfer portal means that that good players that get recruited to top tier schools, but who would - in previous years - languish on the bench behind other top talent, can now transfer out to a different school where they can start and get more playing time. This is the biggest one imo, because it's done more for parity and spreading talent throughout the NCAAs than anything else.
Also, I don't really get the perspective that this is all somehow a bad thing? I WANT to see upsets. It's boring to watch games/tournaments where you know the outcome. These upsets are exciting af