Top dieback like this can be caused by a variety of agents. Bark beetles, stem canker disease, or animals (squirrels, woodrats, porcupines). Close inspection of the stem where it transitions from live to dead will often reveal the cause.

If you can get a vantage with binoculars to see the main stem at the point where the dead tissue starts, you may be able to see signs.

Animal damage may be evident if bark has been chewed off and you can see inner layers, perhaps some sap bleeding below. You may also see the animals themselves at different times of day.

Stem cankers appear as sunken dead or deformed bark, dark in color.

Bark beetles make little holes, perhaps some sap bleeding from holes, perhaps some signs of birds pecking after the insects.

Some root diseases also cause top dieback, but the remainder of the tree crown looks very healthy, so I doubt that.

See what you can see.