If I were to guess, I'd say the Spd stat has something to do it with it. Basch in my party is always berserked (Attack and Speed up), and is the indisputed Combo King, followed by Vaan. Lady Ashe has really only been doing combos since I handed her the Diamond Sword, so I'm guessing it's more the weapon than her.

As I recall, Penelo likes to multi hit, and Balthier has a knack for it as well, though I've never seen Balthier go above a 3 hit.