For at least half that time period between world wars, there was significant economic depression.

It might simply be the economy. The vibes are bad and have been bad since the late 70s because wealth distribution is lopsided and income growth is stagnant.

Question is why was this the case? I feel a natural explanation is it's a reaction to there being less people around on account of the huge number of deaths, in which case there's probably little reason to worry: if birth rate increases as population drops then the system should tend to equalise itself. If it's something else related to a gigantic war specifically then it's a bit less certain (one very important question is was the growth just a displacement in time from all the couples who could not have a child during the war but now can?).