One possibility is simply spring frosts. Grape vines are generally winter hardy and can survive very harsh winter weather but once they start to grow in spring they are actually quite delicate and a hard spring frost can cause a lot of damage.
I don't have a grapevine but do have a kiwi vine. It is similar. When dormant it seems to take any amount of cold, but once the buds start to swell, frost sends the buds black and damages new shoots.
Don't take this as certain. There could be other possibilities. But worth researching further on line.
P.S. I would add that I detest late frosts and cold spring weather. Newly emerging spuds, fruit blossom, and tender plants of all sorts get killed or damaged. For some crops the damage emerges later as the cold tends to trigger running to seed in variety of crops like beetroot, celeriac, leaf beet, etc. A real nuisance!