Sunflowers

The sunflowers died the day after I introduced them into my garden. They went to sleep that night, closing their petals so tightly that the next morning they struggled but couldn't open them up again. The sun was in zenith, but the flower discs remained fisted, and only in the very middle I could see the soft yellow labella and the black eye of the seeds. I wonder what I did wrong. Did I hurt their root system while replanting? Was it too cold for them at night in my yard? Did I water them too soon or not soon enough? Is there any rational explanation for their behavior except for or in addition to their severe dislike of me?