"Your post as Warden-Commander of Ferelden has been filled with challenges and difficulties. As an Orlesian, the local nobility in Amaranthine have given you a frosty reception. Weisshaupt has provided little direct assistance but has recently promised more aid if you complete one small task.
During the last Blight, the hero of Ferelden worked with a purported witch of the Kocari Wilds: Morrigan. The role she and her mother, Flemeth, played in the Blight has caused a great deal of concern to the First Warden. Although scarce on details, the First Warden is offering considerable support if you find Morrigan and acquire some information. What information you need to attain, though, is frustratingly vague. "
-Codex for the OW who plays Witch Hunt
TL:DR The First Warden is worried about Flemeths (a legendary abomination) and Morrigans (spawn of the legend) involvement with the Blight.
This might be because of Flemeths warning before Ostagar, that she saved the Warden, that Morrigan was sent to accompany the Warden, that Morrigan was spotted entering/exiting the Wardens chamber before the battle of the Archdemon or just the fact that a legend like Flemeth was involved in the story at all.
So he sends the local errand-boy/girl from Orlais to investigate. That's why the Orlesian Warden goes looking for Morrigan. And if the Orlesian warden refused, the First Warden sent Jaden (And you don't want that :C).