Mounds of sand? Why would there be sand? Is the soil sandy? Are you saying you can see holes in the ground around the base of the hydrangea, with little piles of something next to them which might be sand?

Mice nesting in the roots? Bees? Wasps? I don't know of anything that attacks hydrangea roots specifically, but quite a few beasties make random holes in the ground.

Holes with little heaps of fine soil or sand could be ants. Ants don't directly harm plants, but if there is a nest under the bush it could have damaged the roots. Most ants will have gone on their mating flight by now after which the workers will all have died so you may have got rid until next year, but it might be worth putting down some ant killer just in case. (Did you see lots of flying ants a few weeks ago?)