On a recent flight from MDW to EWR I happened to sit near the security 'checkers' at the gate. There was a slight delay, and they began a conversation with a passenger next to them. When asked how they 'randomly' selected passengers, they said they usually take those that jump up when the door is opened and the announcement is made for First Class & Elite boarding--since they like to annoy the flyers who 'think they are so important & can fly in FC. They also look around prior to boarding and pick out 'business types' who they know will get upset at the prospect. They did say they generally will not pick out families, moms with kids or those who with lots & lots of stuff to go through.
Now, this is not in any way official, just what I overheard and is obviously not true at all airports.
But after that conversation, I'll never again get up at the first call!