This has been going on for many years. ORD-AMS is one flight where this happens rather often and what they are doing is looking for undeclared cash by going on a fishing expedition for money, using the answers to questions to determine whether or not to search. It's also meant to intimidate, as if that stupid idea generally works against committed criminals.

This is part of one of those unending unwinnable "wars" that the US government likes to fight -- this being the "war on drugs"-related "war on money laundering".

They target certain routes more often than others and stick to it for a while and then move on for a bit and then return.