Lots of things, external like wind, kids, dogs, etc. if related to the vine it could be water, but you should see the leaves drooping and the ends of the vines shriveling. That's all I've ever experienced.
It could also be insect damage as in Grape Berry Moths. The moth lays eggs on the clusters and the larvae grow inside the grape berries. This causes the berries to fall off the vine as the larvae get bigger. Look inside some that have fallen off recently.