If you are using vehicles, then try Mando_Move, which you will force the vehicles to drive exactly the path you want.
If the AI is on foot, you could script it with simple loops that force a walk or run animation in a particular direction. This is the brute force approach, and is a pure scripting solution (ok for a cutscene, or very controlled circumstances).
You could also try the Hoz's doFollowUnit script. One unit will move to a position near the followed unit every few seconds. If you chained this so the Player is the first unit, then unit2 follows player, unit3 follows unit2, unit4 follows unit3, and so on...maybe they would all cross the bridge (with player leading). Not sure if it would work or not (if you are saying AI is reluctant to cross the bridge, this might not work).