The Surprise Round
If one side of the conflict is surprised (or ambushed) by the other, they get a round of their own to act before round one begins. This is the Surprise Round. It doesn't matter what order it is in which each combatant acts, as the surprised party cannot attack during this round.
Whether or not surprise occurs depends on the adventure, not random chance. However, the adventure itself may decide to determine it by random chance.