In tarakna baqiyyatan minhā, tarakna is a feminine-plural past verb whose reference is the plural asfār, “journeys”; such agreement with a non-human plural is an established Arabic pattern. Baqiyyatan functions as an object, referring to what strength remained.
The following fa- introduces a consequence: travel depleted what was left, so the animal’s back and hump lost fullness. A sequence of verbs turns the body into a record of the journey.