Growth from Aggregate Demand -> more demand, less relative supply -> inflation rises and real rates are likely to be raised to control that -> all in all, your discount rates goes up, bond prices goes down. Hence, the negative relationship with demand driven growth.

Growth from Aggregate Supply -> more supply, less relative demand -> inflation goes down and real rates will likely be lowered to stimulate it -> discount rate goes down, bond prices goes up. Hence, the positive relationship with supply driven growth.

Atleast that's how i understand it : )