Lack of care and poor management
Trees have an amazing ability to grow in ways that help to prevent them from breaking apart or falling over in severe weather but, for various reasons, a tree can eventually reach a stage when catastrophic failure could shorten its life. This is a particular problem with trees that our ancestors used to cut regularly until many decades ago. Such trees now have heavy branches that can easily break away. Some form of cutting can enable many trees to live longer by preventing catastrophic collapse, but it can be extremely difficult to obtain the right balance between the two extremes: either killing a tree by cutting it too harshly or letting it collapse for want of sufficient cutting.