It could be a number of things Gavin, such as over fertilising, soil not acidic enough, heat stress, not enough water or too much water, a fungal disease, or even natural dieback as the plants move into the cooler season.
I have seen the same thing in our blueberries and a little more regular water seems to improve this disorder but I would recommend going through a process of elimination starting with what you think might be the likely cause and correcting that to see if it improves the situation.