Jason, it could be something like weather damage (freezing, hail, wind) that lets the leaf streak fungus in, or some small pest like thrips or spider mites.
As Larry, said, there is also natural aging, a leaf doesn't last forever (although I wouldn't expect them to do that en masse very early in the season). Leaves may turn yellow as the plant retrieves nutrients from the dying leaf (which is also similar to what happens in nitrogen deficiency - because plants can move nitrogen around to where it's needed, if it is in short supply it may move it from the older leaves to the newer emerging ones, hence the yellowing appears on the oldest leaves first as the N is translocated).
After flowering some daylilies get pretty ratty looking as well. Diagnosing a plant problem is often a process of elimination.
