I recently came across this tree at work. It is infected with the decay pathogen Ustulina deusta, a very significant fungus that rots the main stem and root systems of trees. It can be very difficult to spot as the fruiting body of the fungus (the bit we see) is small and very discrete. It also can develop just below the soil level against the main stem. It is a constant reminder of the need for detailed inspections of the stem bases of trees as it can be overlooked so easily, although it does seem to have been pretty prevalent on this tree.
Ustulina deusta can result in sudden and catastrophic failure of a tree with no warning or symptoms present in the crown. Many trees infected by fungus rotting their stems and roots will also appear in decline thus aiding identification of an issue with the tree.