Associative hatches are linked to their boundaries, and update when their boundaries are modified. 
Non-associative hatches are independent of their boundaries.
To set hatching to be associative, check Associative Hatch on the Display page of the Drawing Setup (Options / Display). To break the link between an associative hatch and its boundary, select both the boundary and the hatch, and select Drop Link from the Inspector Bar or local menu. 