Question
Why is just being hollow not a sign that a tree is ready to fall? Isn't it dead already?

Answer
No a hollow tree is not necessary a dead tree. The only living part of the tree is the layer under the bark called the cambium layer. The wood part of the tree is dead cells and when a fungi gets into a wound on the bark it begins to rot the wood and over time the tree becomes hollow. A hollow tree does lose some of its strength and could break during a high wind storm. It does not mean it is ready to fall just weaker than a solid tree trunk.