Earlier quoted context omitted.
Why would this be the most likely scenario? What would motivate someone to end their career in this way?
They have principles and wanted to make a statement. But like I said in a different reply - my default interpretation of this given the circumstances would be not considering this as more significant than someone earlier in their career simply speaking out. Would they have resigned if they were only 35? maybe - and that would (to me) be stronger evidence of how bad the things were that they were protesting. And maybe…
One might almost hear a tinge of regret that they aren't retiring now with a nice party at UC Berkeley....