I am not sure how mulch could cause this but can you explain how you supposedly did it again? How do you mulch a climbing hydrangea such that you end up spilling mulch on its leaves? I have a mental picture in which the hydrangea is climbing up a tree or some support structure but most of the shrub (in my mental picture) is above 3-4", the point at which one normally stops adding mulch. So how did you manage to spill mulch on higher up leaves again???

Is the plant getting too much moisture and developing root rot? Or not getting enough moisture so the leaves where already fixing to dry out when you left? Have you applied any insecticide or fungicide near this plant? Is there a tree growing near the hydrangea that could be competing with the hydrangea for water?

Luis