I was thinking they "may" be able to observe dominant vs. Submissive behavior.
Also, I did not say they "always" take a leader first, but that they would take them at first opportunity.
Lt.Gorman may have been a leader in title, but not a good enough one to have the others behaving toward him the way they did Apone. Just because one has the title of leader does not mean that they truly behave as one, or are socially viewed in such a role.
Brett may have made himself a more convenient target before Dallas, and the Alien is not going to pass up an opportunity to catch someone alone. Then again, the same argument could be made for Dallas as well putting himself in the Alien's path.
Same thing goes for Dietrich and Wirezbowski being more convienent, but they did not take long to get Apone after them.
A3, Murphy, Boggs, Rains all had made themselves convenient targets. Again, they will take any chance they can get, but Clemens was a leader in a way, more than Andrews was. Sure Andrews was not and inmate, and was 2nd in command in title, but really no one displayed submissive behavior to him.
As far as Dillon goes, he was fortunate enough to be NOT be as soft a target.
Again, I am not saying that the Alien would only attack them first, but that in general a victim that others are showing submissive behavior toward, with body language and such, that this is like saluting in a combat zone. If you look like a leader you may be a "preferential target."
They seem to get picked off early on, not exactly the first to get it, but still early on compared to the rest of the victims.