I strongly doubt it. Check for squash vine borer holes at the base of the pumpkin vines. If you're stuck on the idea that it's nutrient loss to sunflowers, toss half a bag of compost around the pumpkin plants and water it in (or spray the area with water-soluble fertilizer, fish emulsion, manure tea or whatever floats your boat). It won't hurt anything and will provide enough nutrients for both.

I figure you're having a problem completely unrelated to the sunflowers.