I'll toss what I see as obvious into the mix: Sanders is prone to extremism by personality. I believe he's been demonstrating typical traits for decades of his well documented past. Including his extreme righteousness and conviction, extreme dishonesty, frequently ruthless behaviors, and blanket contempt and convictions of corruption for all colleagues who aren't with him (all of them) and for anything else that gets in the way of his great endeavors, like popular votes.
Extremist thinking must explain away all facts that contradict what is known to be true, leading to what many others see as dishonesty and odd notions but what they see as great truths others don't comprehend. Sanders knows that Democrats are really just Republicans with conceits and hypocrisies of being different. This belief is integral to his own identity and importance, and nothing has ever convinced him otherwise. He's not crazy, of course, and is quick and competent at backpedaling to safe ground whenever an interviewer calls him on something. But, he never changes his mind.
Anyway, the bottom line, whatever's going on with him, is that Sanders won't accept that voters are choosing someone else with the same respect for and conviction in democracy as our liberal Democratic candidates. Whatever his real goal is, eventual revolutionary takeover or to just cling as long as possible to the national stage denied him for 7 decades, we know he'll once again do whatever he needs to to prevail over the decision of the Democratic majority.
And that meets my lay notions of extreme behavior.