To the best of my understanding this is not a "security tactic." Rather, it happens more when the people who are called don't show up. That is, the security people are supposed to be used for constant screening. So, if the randomly selected people haven't shown up yet, they take someone from the line to keep the security person busy. Therefore, this happens less at AA where they call about 10 seats worth of random before any boarding starts. The security person is then so busy that they don't have the temptation to take someone else out of line.