I think the CBS scenario was slightly different because their was build up to it. They were aware that there were complaints against him, and then were aware that he had been suspended, and then terminated. It came in stages and the team there was able to process it and THEN report on it. In this case it seems like NBC wanted to ensure they broke the story and in order to accomplish that couldn't tell the team until this morning.
The gossip reports on what happen indicate that a staffer accused him of sexually assaulting him when they were in Rio covering the Olympics. Details are in Page Six. It says that people at the network knew he'd had affairs with other staff members but that they'd been consensual. This was an issue because it was non-consensual.
As bag-mania says above - there are also reports that the NY Times was looking into this, so NBC had to act fast for damage control, which also expalins them choosing to break the story themselves.
