You'd have to be a board member to know for sure, but I believe Kellner was "invited" to retire for a few good reasons: 1) disastrous handling of fuel hedging over a period of years, 2) failure to reach accommodation with the pilots on scope - thus the DH4's, 3) allowing Biz/First to lose its polish (and profitability), and 4) overpaying for LHR access.

It's easy to say some of this criticism is just hindsight, but the cumulative result is that the company cannot reasonably predict a return to consistent profitability. Somebody had to pay and Mizner's departure wasn't enough.