I haven't watched the bit yet, tomorrow I'll have time, but from the headline I have 0 surprise.

Trump built his entire campaign around Biden, and without it he literally cannot create an argument. Everything is failing against Kamala because she's legitimately a better candidate in most every way compared to him, and I think he knows it. He was a one issue candidate - not Biden. He doesn't know what to do.

I also wonder (and maybe Jon goes into this), if it's a mental thing now for him. He couldn't stand that Biden won, beat him. So he wanted a rematch. By stepping down gracefully Biden had the last laugh. He can never lose to trump. Trump has only lost to him. Since I doubt he's in therapy and knows how to manage these emotions, it is no surprise that he can't stop fixating on him