I am not sure how you have come to arrive at it not happening on channel 1. Are the naga not on the bridge? Is the NPC that gives the quest to capture, then defend the bridge not there? What exactly are you finding that is or is not happening?
On a side note. After you capture the bridge and are given the quest to defend it, it can take up to 2 hours before the naga return to attack it. I have had to teleport in and out from time to time just to catch the attack and finish the quest.