Most of the time, the person who breaks the relationship off has been unhappy/checking out for some time.
It often starts with minor irritations that aren't big enough to split up over. There are arguments (the rough patch or things have been tricky part) where one person tries to address their concerns. They often don't state to their partner that it's potentially relationship ending because they don't realise themselves maybe at that point. Or there are children involved. Or financial commitments.
So they trundle along becoming increasingly dissatisfied and irritated by the other person. This breeds resentment.
In their head, the relationship is essentially over but they can't face the upheaval of ending it so things just plod along. It would usually be clear to the other person too if they would only look for it but they don't because whilst the other person is still there, things must be ok. Both parties are happy to keep their blinkers on, put one foot in front of the other and just keep moving forward.
Until the unhappy person meets someone else and suddenly all the upset, upheaval and untangling finances seems worth the effort.
For the other person, it feels like it's out of the blue (even if they acknowledge things haven't been great for a while) but, for the person who leaves, it's been a long time coming.
It's not just men who do this. It was just a man in your fmiy who did it.
When lives are entwined, it can feel selfish to leave an unhappy relationship. There are threads on here every day that are testament to that. Likewise, it's easy to argue that it's selfish to stay.
But it's not that people moved on quickly once out of a relationship. More that they've already moved on whilst still in it.