So many more things had piled up on her that her nerves were quivering the way a horse's legs will quiver when it is whipped. And then the three year-old himself behaved with that mean, miserable ugliness which unfailingly confirms the doctrine of original sin. This time it pushed sweet mother over the edge. Now she is humiliated. She feels that everyone is looking sideways at her, as though she were uncommonly cruel or unusually unstable, when in fact she is neither.
Somebody has an affair.
Betrayal of one's wife or husband is a very bad thing. But it's not the only bad thing about the affair, not necessarily the worst thing. Think of the utter self-falsification which characterizes the betrayer finally. But he didn't start off sunk in self-falsification. Affairs advance a step at a time, proceeding incrementally.