There's a third, more mundane possibility: he lost his tenure because he quit. When he lost his new job offer, he went back to Wayne State asking for his old job back, and they said no.
Well in fact he did get his old job or position back at Wayne State, Michigan, but at the reduced level of pay, and without the other benefits of tenure. This was most likely simply the administration trying to control their expenses, as they had most likely planned on replacing him with a non-tenured professor.
> Moral: never, ever, quit your current job until the ink is dry on the legal papers for your new one.
Good intent, but often hard to keep in practice while also managing obligations such as the notice period for resignation (14-90 days), and planning (i.e. relocation) - most employers won't continue to pay you while you move away from your place of employment. In terms of selling a house, moving out of state / province, these things are fairly significant events that take time and away from your current job.
I don't know the legally binding nature of a job offer, I suspect it varies by state/ province and by the actual offer, but they do not offer the same protection as a contract of employment, a document that I have normally not been able to sign until the first day of work.