The junior novelization of Star Wars: The Rise of Skywalker subtly reveals the real reason First Order Stormtroopers rebel. Every empire must deal with traitors, but the First Order seems to have a particular problem with them. Finn's treachery in Star Wars: The Force Awakens led to Poe Dameron's escape, while Star Wars: The Rise of Skywalker introduced an entire squad of Stormtroopers who had rebelled.
According to official tie-ins, most First Order Stormtroopers are recruited as children. Delilah S. Dawson's novel Phasma revealed these children are then subjected to a harsh regime of mental conditioning, essentially brainwashed for blind obedience and loyalty. While that presumably works in the vast majority of cases, there is some resistance; in Star Wars: The Force Awakens, Phasma initially intended to send Finn for reconditioning, and she clearly seemed to feel there was nothing unusual in that being necessary. But why is the First Order's brainwashing program so fallible?
The junior novelization of Star Wars: The Rise of Skywalker subtly hints the dark side of the Force itself is used as part of the brainwashing process. It expands on Finn's unease when he sees the Jedi mind trick, and sees him begin to question whether the First Order had used similar techniques.
"What was worrisome was if someone who lacked Rey's principles wielded such powers. Was that what had happened with the First Order? Had Finn's own mind, and the minds of all the children taken to be soldiers, been somehow warped and influenced by a dark power?"
If the Force has been used as part of the conditioning process, then you would naturally expect a minority of beings to resist it - simply because they are Force-sensitives. That fits perfectly with a later conversation between Finn and Jannah, in which Jannah revealed her entire company had spontaneously downed weapons in the middle of a battle. According to Jannah, there had been no conscious decision; there was just a sudden instinct, a feeling this was wrong. Finn, of course, concluded this could only be the Force - and he was probably right.
This has profound implications for the future of the Star Wars galaxy, because it's generally assumed Rey will rebuild the Jedi Order. She already has one potential apprentice in Finn, but it seems she would be wise to turn to Jannah and her company of former Stormtroopers as well. Viewed in that light, it's hard to avoid the conclusion the Force has already gathered together Rey's first generation of students, ready and eager to learn the ways of the Force. Presumably any other Stormtrooper rebel has the potential to be a Jedi as well; Phasma's comment about the Stormtroopers needing reconditioning may well imply there are hundreds of potential Jedi out there.