The more I read about it, the more it reeks of a baseless smear campaign. CNN actively tried to make Biden look bad during the debate, and a huge chunk of the corporate media saw it and jumped on it.
It helped when Biden turned right around and had a strong showing at one of his campaign rallies. He had one off night, and everybody tried to make a mountain out of a molehill because they're more concerned with making money than fighting the fascists.