Trees can be tricky as sometimes a tree will look very healthy, having lots of leaves and not much deadwood, but there may be something happening inside the trunk. The trunk holds up the branches, and has to be strong, but if it has a certain type of fungus inside it, it may look strong on the outside but be weakening on the inside. Often, it is impossible to tell if the tree has a fungus inside until a little round piece of wood grows straight out from the trunk. This is called a bracket fungus.
When this happens, if the fungus is a certain type, the tree has to be removed or the trunk might collapse.
Not all bracket fungi are bad though.
Sometime a tree will have lots of deadwood. This usually means the tree’s health is suffering, and more investigation is needed. Sometimes trees like this can be repaired so there is no need to remove the tree.