> I had never received any feedback indicating my performance was poor. In fact, it was exactly the other way around: I’ve never had a role that was such a good fit for my expertise before. So I asked if he could detail better where I failed. He refused to..
It is completely unprofessional to fire someone having never given any feedback. Startup or not.
And providing reasons after the fact, as others have stated, is a bad idea because it opens up liability. You have no discriminatory reasons whatsoever for letting someone go, but you bumble some words and the next thing you know you're getting sued. Better to treat it like you're talking to the cops and just not say anything.
I agree there should be better feedback. If they are annoying, have the decency to tell them why and point out a few things so they don't go from job to job wondering why they don't fit in.