There is no textual evidence to show why Patrick ends his marriage. He comes home from work and is cold and distant. This could be his usual manner or it could be an anomaly. The narrator notes that Mary "loved the warmth that came out of him when they were alone together." So, either he's being typically cold or he's being atypically cold. It isn't until he repeatedly tells her to sit down that she begins to think something is different.
After she finally sits down, he tells her that what he has to say will come as a shock. The next paragraph opens with him saying, "So there it is." Then he adds that he will give her money and that it won't be a big deal. The bottom line is that we don't know Patrick's motive to end the marriage. Maybe it is Mary who's been unfaithful. This would explain why he is so cold and why she is so attentive, as if she's making up for some mistake. Or, it could be that Patrick has been unfaithful. Again, if he has fallen in love with another woman, his detachment makes sense. And Mary, even though she's been cheated on, is acting out of desperation to keep their marriage together. Or, it could be some other scenario. There's no way to say for sure.
The author, Dahl, teases and coaxes the reader to look for clues, anything to shed light on the motive. This is fitting because the policemen who arrive to investigate Patrick's death are incompetent and they unwittingly eat the evidence. But just as Mary hides the evidence, Dahl leaves no evidence for the reader as to Patrick's motive.