Why did the Knights of Ren attack their leader?

He shoots some kind of Palpatine-affiliated guard in the shot before he encounters the Knights of Ren (sorry for the blurry image):

Since the Knights of Ren are apparently working with Palpatine at this point (for unexplained reasons; possibly because the rest of the First Order has been subsumed by the Final Order), they probably saw or heard this happening and correctly surmised that Ben had gone rogue.

Additionally, the Knights of Ren were Force-sensitive to some degree, according to the Visual Dictionary. When Kylo Ren approaches them on Exegol, he's (apparently) already turned back to the light, or at least doesn't seem to still be consumed by the dark side. Since they had spent a good amount of time with him, given that he was their leader, they may have sensed that something was wrong.

In the canon comic Kylo Ren, the Knights of Ren already operate under Supreme Leader Snoke. It is possible that they were always under the control of Palpatine, given some information about Snoke that we learn in the movie.

Kylo Ren certainly thought that he was their leader and that the Knights of Ren were personally loyal to him. It's not until he arrives at Exegol and finds them guarding Palpatine's shrine and waiting to attack him (presumably forewarned of his treachery by the Emperor's prophetic dark side visions) that he realises that he was being duped. They were never loyal to him or Snoke, they were Palpatine's creatures and always had been.

He reached the end of the Sith monoliths and rounded a corner. Familiar figures manifested in the flashing dark. First Vicrul and his scythe. Then Kiruk and his plasma blaster. And suddenly all six were arrayed before him. His Knights.

For the briefest moment, Ben actually thought they’d come to help.

But hate rolled off them in waves like fetid air. The Knights of Ren had never been his. They had belonged to the Emperor all along.

A final betrayal.

Snoke had been nothing more than a pawn. The Emperor had whispered poison to Ben his whole life. Now even the Knights, those whom he thought his faithful brothers, were raising their weapons for the kill.

_{^{Star Wars: The Rise of Skywalker: Expanded Edition}}