I kinda think it might be time to fertilize, which I haven't done in the 8 years I've had vines, but there are no signs of P deficiency or excess N for that matter since those issues can cause coulure. I highly doubt it was disease either. I was actually on the ball with it this year and started spray at budbreak but there was some anthracnose. I'll post some pics. I have heard that excessive warmth can cause it. If there's no other cause, I'd bet on the warmth.
EDIT- After writing that, I recalled that I got hit early with herbicide damage. Everything that showed foliar signs of damage lost their clusters. I wonder if the herbicide directly or indirectly caused this. It was a strange thing too. 2,4d and dicamba (by far the most popular herbicides here in IN) only affect certain vines and there were vines damaged that don't get affected by either. Very curious.