How come I was asked by a CBP officer why I was in the US when leaving?

CBP occasionally conducts spot checks of departing travelers. They may be concerned about immigration status or about travelers moving cash out of the country in amounts large enough to trigger the reporting requirement without having reported it. Since customs and immigration are now handled by the same agency, a single officer can investigate both. Questions about the purpose of travel are part of such an investigation.

I've seen this mentioned on various forums and depicted on reality TV. It appears that they tend to target certain flights, perhaps because of a (perceived) greater likelihood that those flights will be transporting people likely to violate the cash restrictions. For example, passengers on flights to countries where money laundering is common are apparently more likely to be screened.

Another possible explanation is that CBP is screening departing travelers in connection with their work on a unified biometric entry-exit system. Having data collected by an officer may be useful in helping them calibrate their algorithms for automated exit screening. For example, this was a test conducted a few years ago:

• Test To Collect Biometric Information at Up to Ten U.S. Airports (“Be-Mobile Air Test”)

Here are some relevant proposals for new federal regulations:

• Collection of Alien Biometric Data Upon Exit From the United States at Air and Sea Ports of Departure
• U.S. Citizenship and Immigration Services Biometrics Collection for Consistent, Efficient, and Effective Operations
• Collection of Biometric Data From U.S. Citizens Upon Entry To and Departure From the United States
• Collection of Biometric Data From Aliens Upon Entry To and Exit From the United States

People sometimes state that the US has no exit immigration. In practice, this couldn't be further from the truth.

The US uses a number of mechanisms to police exit immigration. These include electronic checks based around data provided by the airlines (via APIS, the Advanced Passenger Information System), and a number of various random checks that are carried out... well.. randomly.

What you experienced falls into one of these categories, but there's no way to tell which. It's possible that this was just a general random check, looking for nothing and anything at the same time - suspicious passengers (who might be carrying contraband), forged/modified passports, etc.

It's also possible that the electronic checks via APIS data flagged a particular passenger who they wished to detain. As this data is provided by the airline it is not possible for the person to be detained at check-in, so instead they are allowed get to the boarding area where they will be detained. In this case the officers would have had a specific names and/or photo they were looking for, but still check other passengers as a matter of course.

It's even possible that something illegal was found in a passenger checked bag during screening, and they are looking for the bags owner.

As you've discovered, they position themselves in an area where they can not be seen by the passengers, giving them no opportunity to simply leave the area without being checked. By the time the passengers see the staff they are unable to leave without going back through the check-in area - an action that the CBP staff would see and act upon.