Yep..they're dying...it's what they do.

Yeah, these happy-delivering flowers meet the dark lining of the cloud by looking incredibly sad when they're heavy with seed and begin to fade away. You've quite a bit of seed there!

Succession plantings are keeping sunflower happiness around though the flowers seem smaller than the spring-planted ones.

It is part of the circle of life. At this point, if you want, you can cut off the seedheads and set them in the shade somewhere to finish drying and maturing. I line them up on a table on the patio, or if it is rainy and misty (I wish it were!) on a table or shelf in the garage. If I leave mature heads on sunflowers, the deer come along during the night and eat them. You may not have a deer problem like I have though.