Model this East German crime

Following the collapse of the communist regime in 1989, the number of births halved in East Germany. These cohorts became markedly more likely to be arrested as they grew up in reunified Germany. This is observed for both genders and all offence types.

Here is the full article by Arnauld Chevalier and Olivier Marie, the authors seem to think the reunification event selects for risky parents, are there other possible explanations?

Via the excellent Kevin Lewis.