Virus is not suspect based on their symptoms, but root disturbance might be the reason. Could voles be tunneling in that area (they chew on plant roots and sometimes pull whole plants into the ground to feed on them)? Or might squirrels or chipmunks been digging at their base, disturbing soil? (We've seen this happen with squirrels and Hosta grown in containers, for example, where the potting mix is appealingly easy to dig in.) Is the soil moist enough? Aside from recent rains for some areas, the past few weeks have been fairly dry. Root competition from the nearby, shallow-rooted azaleas, and the aggressive Ajuga groundcover, might be contributing to root moisture stress if that's a factor. Feel the soil about five inches deep near the Hosta and water well if it's becoming somewhat dry to the touch at that depth. (The azaleas would appreciate the irrigation too if it is that dry.)

For now, just keep an eye on them. If voles are present, you might need to resort to trapping them if deterrents like physical barriers to digging aren't effective. They don't live long lives and many animals prey on voles, but in the meantime their damage might be substantial (among herbaceous perennials at least) if you wanted to wait them out until a resident vole died naturally or moved on.

Miri