There could be a wide variety of reasons. Stress from root damage (any construction in that area in recent months?) or dry (or too wet) conditions, symptoms of a pest or disease presence, or squirrels chewing off branch tips (though leaves on the ground likely would include the twig tip). Do you have photos of the trees? It's hard to guess without more information and images of the symptoms. A certified arborist could also take a look at the trees to check for indications of damage, stress, injury, pests, etc.

Miri