Kharajtu is a first-person perfect. Bihā associates the woman with the departure. Amshī is a first-person imperfect and tajurru a third-person feminine imperfect; both depict ongoing action within the past narrative.
Tajurru belongs to the root j-r-r, “to draw or drag.” The woman is its subject, and dhayla mirṭin functions as its object. Two agents are thus shown in one scene of departure.
Without amshī tajurru, the verse would merely report that they left. The imperfect verbs convert the past event into a moving scene as the cloak trails behind.