I made my own little embark with 7 dwarfs each with mining 5 only, yet one of them was titled "Expedition Leader". Is he chosen randomly or based on his traits (and stuff, since i dont really keep track of my dwarf's attributes)?
If one dwarf has particularly pronounced social skills, they will be the Expedition Leader. Absent those, the highest administrative skills will get the job because nobody else wants it.
In the case that no dwarf has any social or administrative skills, the leader is chosen at random.