Are the canes dying before they fall off? Are dying branches still connected fully to the main vine? It is not uncommon for young grape canes to break off early in the season, particularly if a storm came through. 

Break off a cane that is dying but still attached, and slice it lengthwise. Look for any indication of an insect boring into the stem, such as the presence of larva or frass. Then, please upload a couple of clear photos so we can see what is happening inside the cane. Also, please provide a photo showing the grapevine on location, so we can see healthy and affected canes in the photo.