That's less fiduciary duty and more hacking a payday.
A large golden parachute for firing a board member could be a disincentive mechanism to do it: a 'poison pill' of a kind.
* https://en.wikipedia.org/wiki/Shareholder_rights_plan
They could have felt they were doing the right thing, and making a public statement of the situation, but knew it was a bit of a kamikaze tactic and so made it more painful for when the eventual backlash occurs.
If they truly believe the CEO was destroying the company/its value.