At least in the case of Klaus Fuchs, he was a member of a family active in communist politics in Germany before emigrating to the UK and then the US. Others are

At least in the case of Klaus Fuchs, he was a member of a family active in communist politics in Germany before emigrating to the UK and then the US. Others are first generation born in the US, but at least in Seborer's case, the entire family seems to have been active in communist politics. The development of communist and socialist ideologies in the German, Austro-Hungarian and Russian empires prior to WW I provided a very fertile ground for recruiting among families who had fled repression in those regimes.