Thank you, Denise, for your question!
    A first response I have is to recommend a soil test for your garden.  This website Home | MSU Soil Test outlines how to order and how to sample your garden to send in a soil sample. 
  My initial inclination is that there is too much ammonium from applied fertilizer in your soil, more than your plants can take up and utilize.  Low soil pH along with wet conditions can cause your plants to respond in this curling formation. Soil pH impacts how nutrients in the soil are taken up by the plant. We have had a significant amount of rainfall that can add to this scenario. 
   Your soil report will deliver soil nutrient values. When you send in your sample you will have noted this was from a flower garden. The results will then outline how to correct your soil conditions to make your zinnias happier and perform well. At the end of this growing season, you may wish to add organic matter to your garden to help aerate the soil.
   Congratulations on such a huge garden of zinnias! 
For further reading:
Ammonium toxicity can cause chlorosis in pansies and other crops - Floriculture & Greenhouse Crop Production (msu.edu)

Ammonium Toxicity: Avoid Getting Burned This Winter (growertalks.com)


Aster yellows | UMN Extension