Reasons good friends of mine have gone low contact with their former fast friends (other than my own story earlier) include:
Disloyalty or disclosure about very personal information told in confidence. (FWIW I bet this has happened here).
Differences in child-rearing approaches, especially discipline methods.
Diverging political views especially about immigration and Brexit, reproduction rights, and minorities.
Perceived rudeness to partner, family member, nanny or housekeeper or overfamiliarity.
One party is very happy and their happiness throws your own unhappiness into high contrast (FWIW I suspect this might also be a factor here).