If you don't want to smear Sam in public you don't cite him not being candid as the reason he's leaving. They certainly could have communicated better, but mos

If you don't want to smear Sam in public you don't cite him not being candid as the reason he's leaving. They certainly could have communicated better, but most of the actually good reasons are stuff you wouldn't expect a board to disclose, like "we told you not to do this large unannounced deal with a publicly listed company that we intend to totally renegotiate" or or "you lied about how much this tech sucks" or "you keep lying about this in-progress initiative we haven't announced yet and don't intend to launch in your departure notice"