Most VAV systems require that a minimum duct static pressure point be maintained to ensure operation of the air terminals. This means that even at very low flow rates, an elevated static pressure needs to be provided, above that expected by following the traditional system curve. As the fan speed drops to match the required flow, the intersection of the actual system curve and the fan curve moves the fan closer and closer to stall.
What is the effect of this interaction? And how can you select a fan to best deal with this problem? Energy Labs has done a valuable study of this effect and it can be found here.