Trees, bushes and shrubs are often planted under poor circumstances.
One of the problems is that the water cannot reach the roots effectively, or not enough water can be given. Think in terms of trees in streets and pavements, for example. Watering them is an expensive business and will therefore be done as little as possible. When they are watered, the water generally sinks very quickly into the subsoil, particularly on sandy ground.
This causes serious dryness, which cannot be compensated by watering once in a while.