The entire Biden campaign has been incredibly unusual (historically speaking). He was mostly irrelevant in the early primaries, then one day just became the front runner and never looked back. There wasn't some lynchpin debate performance, interview or issue that you can point to. It just flipped overnight through circumstance.
Then for the main election, he hasn't had to do anything except lay low while Trump repeatedly punches himself in the balls. In most elections, candidates are desperately trying to "win the day" the entire summer long. Joe has went the other way and it's working remarkably well.
I can't imagine a scenario like this ever playing out like this again in our lifetime.