Sparse foliage or dieback from the tips is always a sign to be concerned about. Trees rely on their leaves to convert energy from the sun, so if the tree is cutting back on its leaf production, there could be something seriously wrong.
There is a very close relationship between the size of a tree’s canopy and the size and condition of its roots. Signs of ill health in the canopy may thus be the first indication that there is a problem occuring underground. Recent disturbance to soil levels, compaction from parked cars or a well used thoroughfare, waterlogged soils or even gas leaks may all cause root damage and consequently exhibit symptoms in the canopy.
Equally dieback, particularly in mature trees, can indicate the presence of decay fungi, although a thorough investigation would be necessary to confirm this.
Sparse foliage and dieback in the crown of a weeping willow (Salix x sepulcralis)