The exact cause of Spring Sickness remains a mystery, and it appears in daylilies throughout zones 3-9 in the United States. Typically the entire daylily clump is not affected -- usually it's only a couple of scapes. There is not a definitive answer for why this happens. A fungus of some kind is suspected, but as of yet it's unproven. It is also worth noting that daylilies are randomly affected. Just because a cultivar has Spring Sickness one year this does not mean it will be affected next year, or ever again for that matter.
To read more about this phenomenon: http://web.ncf.ca/ah748/qanda.html