Crossfit is full of pulling movements like rowing, muscle ups, and pull ups, as well as movements that require alot of upper and midddle back strength, so its essential to build a strong back.
Exercise: Weighted pull up drop sets 3x max reps w/25lbs (ladies)/45lbs(Guys), after failure is reached on a set immediately do as many Bodyweight pull ups as possible, and do that on all 3 sets.