Spitballing here…
Biden plans on a virtual inauguration. Is this even constitutional? I mean, unless he's physically standing in front of the Chief Justice taking the oath, is it even valid? For all we know, Biden could be a CGI on a screen. If he is not PHYSICALLY present then there would technically not be any witness to the event.
I could see them airing his inauguration anyway, claiming he's President. Meanwhile Trump is still President because Biden't oath is illegitimate. Now we have 2 men claiming to be President of the United States, at the same time. We'll just have to see which one the Military reports to…