My current hypothesis (now that the crib thing is blown out of the water) is that it has partly to do with the family oriented trait. In the code it has a 'random weight' which I believe is the chance that a sim will get that trait when they age up or when the game spawns more sims. I think, with the addition of more traits, the chance for them to get that trait goes down for every EP. The new ones don't add much, but it just might be enough to cause a problem.