5% is probably just normal leaf loss,though they shouldn't be 'rotted' (which implies soggy) they should just go black/dark brown and fall off.

More significant would be the stems going black too. Keep an eye out as this could indicate one of two things. A necrosis causes by an bacterial agent that can be progressive and gradually kill the whole bush. It appears to happen after a cold and very wet winter causing bark at the base to split allowing the pathogen in. The second double whammy is over saturated soil causes the roots to rot and branches start dying off by turning black.