http://wiki.guildwars2.com/wiki/Rescue_the_villagers_and_defeat_harpy_kidnappers
I have figured out how to get this to work. You need to first free all the captives. Let them run on their own and get stopped by the harpies on their way out. The best way to do this is to start from the top and work your way down freeing hostages. they should all start to clump together toward the end.
The problem is when you free a captive and they run out of the area into the ebonhawke guard or whatever, the last one bugs and doesnt leave. Best to start with all the ones at the very end of the event area and then work your way down. It’s ok if they get stopped by other harpies.
Hope this helps.