I've had to fire people before. One time it was performance, but performance can be very hard to measure sometimes, so all I had was a vague "making too many mistakes" etc. This person had been on a PIP for a while, and had over a year to get it, but it just didn't work out. But I didn't have anything more concrete. The other one was literally just "our boss told us we had to fire one person, and it's you. Sorry. No better reason."

I feel like companies should just be more upfront about things like "hey, we over hired. This sucks. But you're fired. Here's a generous severance."