I was on a flight a few weeks ago from SJU-SDQ where over 1/2 the pax (virtually all Dominicans) were subjected to additional screening at the gate. I suspect this was due to the fact that:

1) many of them were probably infrequent flyers; and/or

2) had paid for their tickets in cash, since credit card ownership is less widespread in the Dominican Republic.

I thought it was a little bit absurd, but that seems to be a common theme in the new world of airport security.