I suspect trouble at the root - inspect all branches, and especially the trunk of the plant all the way down to soil level, closely examining at soil level in particular. You're looking for soft areas, splits, cracks, weeping, blackening, anything like that. Also check for honey fungus mushrooms nearby from now and during the rest of this month, particularly if you, or next door, have any dead wood in the ground (stumps from previous trees or shrubs).

Thank you. I will examine. There is a stump of an old tree, but it is at least 4 feet away

A dead stump only four feet away might mean honey fungus, so keep a close watch for honey coloured toadstools in the area - they'll probably be late this year because of the weather. The shrub in front is a Pittosporum, and these are relatively resistant to honey fungus, so the fact that's healthy doesn't mean much I'm afraid.