Someone purposely messed with some medication he was on, that is my gut instinct.
Watch him as he walks onto the stage and he is speaking to the moderators, he was already out of sorts. One of his pills was replaced with a placebo or something intentionally to perhaps increase his condition, I don't know.
They realized he was going to lose by a landslide so they needed to replace him but he was refusing to go. Listen to the comments Pelosi made afterwards as she was happy Harris replaced him. Bidens last move of defiance was to quickly endorse Harris to restrict Democrat options.
We are living in the craziest of times. Biden was just spiteful enough with a bit of that Irish temper to stick it to the Dems. Remember him putting on the MAGA hat? Remember him making the comment about Trumps voters being garbage?
The Harris team avoided him at public events for awhile if you don't remember. There was clearly anger from her camp.
That is my assessment of that. We may never know the truth because even if all of the above unraveled just as I claimed there would be strong resistance.