She cheated on you, not suddenly, but a while ago. And realizes it'll eventually come out, and can't handle waiting for the hammer to fall. So she probably cheated again, or found someone to convince herself that she wanted more than you anyway. She hates herself. She projects that disgust of herself onto you because she can't face who she is and has to lie to herself. I don't know you or anything about you, but I'm 95% sure that's the gist of it. Seen it dozens and dozens of times.

Bad news: You'll never know. See the post title above.

Worse news: It might happen again, similarly without reason, (sometimes people get spooked by things going too well, let alone making mistakes, actually being incompatible, etc). And you'll think, how can I prevent this from happening more? And, there isn't really a way, that's the inescapable poison of never getting closure.

Good news: You don't have kids, aren't married, didn't waste the next 5-10-15 years of your life with her and THEN have this happen. She did you a bigger favor than you probably appreciate at this point in your life and you'll look back at this some day with a "Fuck yeah, bullet dodged" sentiment while you go about your life being proud of who you are.