Spring sickness happens even when there is no frost after the leaves start to grow. The damage starts while the shoots are still underground but it's not clear exactly when it starts, it may even be the year before.
The only website solely about spring sickness is this one which hasn't been updated in a while but shows some background, starting with this page that suggests reasons for some suspects:
http://web.ncf.ca/ah748/2002.h...
If you're an AHS member there was an update article on spring sickness in the Daylily Journal last year, I think it was in the Spring 2018 issue.
