In a sense, the exact reason for Patrick's desertion of his loving wife isn't actually very important for the story overall. What is important is how this news turns Mary from an apparently loving wife who only lives for her husband's happpiness to a cold-blooded and rational killer. At the same time, from the way that it is written, Dahl strongly suggests from what he writes that Patrick is leaving her for another woman:

“So there it is,” he added. “And I know it’s kind of a bad time to be telling you, bet there simply wasn’t any other way. Of course I’ll give you money and see you’re looked after. But there needn’t really be any fuss. I hope not anyway. It wouldn’t be very good for my job.”

Although there is no definite reference to another woman, it is clear that Patrick is leaving her, and naturally the reader infers that this is for another woman. Either way, what is important is what happens next as Mary, in an absolute rage, unthinkingly kills her husband, who was the object of her affection. Dahl is gently poking fun at loving relationships, suggesting that beneath a veneer of kindness and love there may be darker desires and surprising emotions lurking, that if unleashed can have devastating consequences.