My working theory is that Biden will drop out days before states were planning to mail out ballots.
In CA, the DNC has until Oct1 to name their candidate, but the mailings were supposed to start Oct 4th.
From a practical perspective there is no way to print (or reprint) those ballots that fast.
So the state will claim a delay in mailing.
This will naturally result in late ballots and late voting.
Which is exactly the chaos they want.