immature muscadines falling off

Any muscadine growers out there? I am growing Ison, a higher lattitude variety of Supreme. My vines do great every year. Highly productive, very little disease, etc. Its July 28. I just went out and looked at the vines, and there are immature grapes all over the ground! Perhaps 20% have fallen off! If I gently shake the vines more fall off. We have had sufficient rain. No big differences from years past. I see no fungal issues. Perplexed. Looking for an explanation. Thanks for any offers.

Where are you?

Are you in a drought like most of the south?

No, we have had sufficient rain, so thats not it.

Are the stems of the clusters turning to wood?

Not sure. I will check. What does it mean if they are?

If they are, the mother plant has given up on the grapes.

Berries falling off is termed "shattering". Google for grape bunch shatter or similar term.