If you think that's bullshit, you're not considering all the possible factor, or don't understand how a decades-old marriage works: through lots of compromises,

If you think that's bullshit, you're not considering all the possible factor, or don't understand how a decades-old marriage works: through lots of compromises, that people sometimes feel unhappy about, but not enough to risk what they've gotten used to. So maybe the mother had felt (probably for years) that the father wasn't appreciating her enough. And now he's making more effort to connect with this unknown child than he's spent on his wife and other children in years... suddenly the pent-up dissatisfaction comes out into the open, grievances are aired and people say hurtful things that make reconciliation impossible...