"Collapsing"? Did the stems bend due to -maybe- the wind and the weight of the blooms? Can you post a picture of the shrubs? Another picture of one of these collapsed stems?

Until the plant matures the weight of the blooms can take them down. Esp. if they have been heavily rained on.

When my ES hydrangeas don't get enough water, the blooms won't hold together tightly, which may be what you're talking about? Watering generously(I can put 1/2 - 1 gallon on my 2' x 2') will bring them back - but may take a couple hours before the the blooms hold together nice and tightly.