It's hard to tell the cause, but it's not an issue of concern for overall tree health, and enough acorns will remain for wildlife this autumn. The twig stubs look chewed, so are probably cut off the branches by squirrels or insects like Twig Girdler or Twig Pruner beetles. (It is a bit early to expect seeing these two beetles in particular, but we can't rule it out.) Squirrels need to gnaw periodically to wear-down their ever-growing teeth, so that might explain why they would chew off twigs or other debris without actually eating it or using it to augment a nest.

Miri