Wow! What an unusual growth. I also have never seen anything like it. To do more research I would like more information. What type of hydrangea? Can you send a closeup of the growths? When you open one of the buds is their vegetation inside? I do not think it is a disease or a  pest. I suspect it is rampant, weird growth perhaps from fertilizing too much or from a summer of heat then rain. Gardening and plants provide so much entertainment as we never know what they are going to do next. Keep growing, Marianne Binetti