could it be that sessions fired mccabe because he knew if he didn't, he himself would be fired?

[The Velveteen Ocelot]

8. It's almost certainly true that Trump would have sacked Sessions

if Sessions hadn't sacked McCabe. Whether he did so to protect himself or Mueller is another question - I think Sessions really likes being AG and doesn't want to give up the job because he gets to implement some of his favorite right-wing policies. You'd think Trump would be delighted to have him as AG because they are both racist, sexist xenophobes. But Sessions made the mistake of actually following the law and recusing himself from the Russia investigation, and Trump hasn't forgiven him. Sessions has no particular motive to protect Mueller other than maybe getting even with Trump for being such a dick by making it more difficult for Trump to fire Mueller. It's all Game of Thrones. I'm waiting for someone to turn up dead - it's getting to be about that bad.