I don't know. I think cheating (or a desire to be with someone new) is obviously a possibility and I'd probably be suspicious too, but it's not the ONLY possibility. I assume if you are not involved in this relationship, you are not getting a full picture. Presumably if the breaker-upper does in fact care deeply for the other person they are attempting to protect that persons feelings, and are intentionally not raising every single petty little issue. Often a bunch of small things are not by themselves sufficient reason to break up, but seem bigger when placed all together. Maybe person X is incredibly avoidant and does not wish to argue about the little things.
If they were planning to marry and have a family, it may be that person X decided it was time to shit or get off the pot. Maybe person Y was always a placeholder for them, someone that was okay for the moment, but that they never saw as marriage material and now that there is pressure to commit, they realized they don't want to. Maybe person X is afraid to grow up and knows that Person Y is not willing to wait any longer. I imagine the prospect of marriage as the next logical step is probably the clock this person is working against.
Maybe they both became complacent in the relationship and Person X decided that they wanted something else that they didn't see happening with the other person.
Maybe the relationship has gotten boring.
Maybe there has been a loss of physical attraction over time or a sexual incompatibility and person X believes that means that they are no longer in love, or they are not willing to compromise on it.
Maybe the things that they had in common with at the beginning of the relationship are things that the person no longer values/sees as important. 8 years is a long time. The seven-year itch is a known phenomena. The honeymoon period has worn off.
Maybe something else has happened in the other persona's life that has them rethinking their goals/plans in life (death of a parent, a sibling's divorce, watching friends become parents, etc).
Maybe partner X is depressed, battling with substance abuse, or going through some sort of mid-life crisis.
Maybe they have differing views on money/finances, spending habits, or wildly disparate incomes and the person has decided that this is a fundamental incompatibility that is unlikely to change (or is going to be exacerbated by marriage and children), and they are not willing to commit for life as a result.
Maybe one partner has been living a lie for years and the house of cards is about to collapse/be exposed and they would rather ghost than deal with the fallout and have to answer for their many lies (I literally know something this happened to - she was married to him for years before she learned that he didn't have the job he said he had, and hadn't even finished the professional school he had claimed to be in at the time they had met, - it's a truly TRULY wild story for another time).
Maybe partner X has started listening to redpill podcasts and has become brainwashed about how relationships should look and wants a "trad-wife" (sorry, part of me is being silly now).
Maybe they did meet another person that made them feel something, and even though they haven't crossed any lines, it made them re-think the current relationship. In this instance, maybe the other person is hypothetically waiting on the sidelines, but I don't think that necessarily makes person X a cheater.
Ultimately I do think that a lot of the time, people don't leave okay relationships without some sort of push/external reason. Maybe they should, I don't know. I imagine there is more to the story than this.