We suspect squirrels are the culprit here. This behavior would not be unusual for them, and the symptoms of branch tips falling intact don't match any typical holly pest or disease. Are the ends of the twigs on the ground snipped, as if cut off at an angle? If so, that supports the guess that squirrels are responsible. We can't say why they would become a problem now versus prior years of presumably causing little to no damage, though populations are always in flux, and if someone nearby was feeding birds (or squirrels deliberately), maybe that's drawing more to the general area. Squirrels not only harvest twigs on a variety of plants to amend their nests ("drey"), but they also chew to keep their ever-growing teeth worn-down.

If squirrels are responsible here, we doubt there will be any reliable way to deter them. You could experiment with using a faux owl predator statue to scare them off, though they might get accustomed to it over time. A repellent spray is impractical for a plant this size. Fortunately, hollies tend to rebound from even drastic pruning very well, so should fill back in nicely over time, though root and light competition with the adjacent tree could slow that down a bit.

Miri