If we’re going to go all conspiratorial, here’s my theory:
Both campaigns are dealing with old men with diminished faculties.
There’s some drug cocktail(s) that both campaigns have been using to pep the doddering old farts up for public appearances.
If you’ll remember, very shortly before the debate, the accusation that “Biden’s on drugs” made the rounds, and Trump made some noises about demanding a drug test.
For some reason - possibly fear, possibly determination in the face of a challenge, possibly a subtle communication that the Trump campaign had some hard evidence they would, if pushed, release publicly - that led to the Biden team withholding his customary drug cocktail.
Trump, meanwhile, was dosed to the eyeballs.
And that was the contrast we saw - Trump was on drugs, while Biden, for whatever reason, for that night alone, was not.
Remember - for the Republicans broadly and especially for Trump, every accusation is a confession.