We've seen examples of this from other areas (an Ask Extension question from Oregon, for example, at the same time of year), and it apparently can be normal for grape stems to have quite weak connections between younger growth and older growth, to the point where it doesn't take much force to knock them off. Perhaps a wild animal scrambled through the vine and dislodged a few stems. If the plant overall appears healthy and no dieback progresses down the stem, it likely isn't an issue of concern. Still, feel free to send additional photos if symptoms worsen over the  next couple of weeks, and we can reassess the issue.

Miri