Keith's downfall is a long story, but the cliff notes version is this:
He got caught breaking one of the rules at MSNBC (you can't make secret off-the-books campaign contributions to politicians) and got suspended. Then his father suffered a major health crisis and he had to take time off to deal with that, as his father's condition then took a turn for the worse and he died. Keith was given bereavement time off, but immediately started milking MSNBC producers over it to miss work. This was in part due to the butt-hurt over getting suspended but also, due to the fact that A. Keith had gone to bat to get Cenk and the Young Turks a show on MSNBC only for Cenk to fuck it up and get blackballed from appearing on the network ever again and Keith taking Cenk's side in the debacle and B. Rachel Maddow, who was Keith's fill-in host, was given her own show only for Keith to kind of kneecap Rachels' ratings by constantly taking time off, which pissed Rachel and certain network suits off.
The last part was the big one, as MSNBC had a shift in management shortly after Rachel got her own show (the previous management didn't want Rachel to have a show but Keith went to bat for her getting it) and the new management was pro-Rachel Maddow and more to the point, came to the conclusion that Rachel was a safer bet than Keith, who was by that point constantly missing work (which in turn caused ratings to collapse on the prime time line-up) and demanding MSNBC pay money out for the right to read short stories of major authors on the Friday editions of his show.
The new management finally said "fuck that shit" and fired Keith. Keith recovered slightly in that the then rebranded network Current TV, run by Al Gore, offered to give him a new show as their flagship but he fucked that up because Gore screwed him over on the budget of his show and he started missing work, leading to them firing him and Current TV crashing as a result within about six months.
The last bit basically killed Olbermann's "legit" news career and he had to crawl back to ESPN. Keith is basically blackballed from MSNBC so long as Rachel is there and CNN ALMOST hired him when Trump got elected but backed off because they feared that he would be too fucking psychotic in his anti-Trump rantings even for CNN at the time. And it's been joked that his Youtube channekl was funded by Obama era agitprop money, which is why it abruptly ended (IE the money ran out).
He's basically an example of a guy who had it ALL then lost it due to arrogance and hubris.