Nurseries will use grafting as a way to produce trees.  Grafting is when you take a pencil size clipping of a plant with a great attribute and graft it onto a more generic root stock of the same genus.  This is done to create weeping plants, dwarfs, variegated, a better flower color, etc.  The problem is sometimes the plant rejects the graft. This usually happens in two main ways; The first way is the plant reverts to the parent stem.  Meaning the more generic root stock of the plant will send out shoots that may eventually over take the fancy variety.  Most of the time plants are grafted at the bottom of the trunk.  Sometimes, especially in weeping trees the graft will be higher often called a standard trunk.  The second way grafts fail is as the tree grows and the trunk increases in diameter it loses its ability to bring water and soil nutrients to the leaves and food from the leaves back to the roots.  The tree is girdled as if you cut into the trunk in a circle around the trunk injuring its vascular system

More often than not the grafts work but when they fail it can be messy and hard to repair.  Please view the pictures below that make this concept more obvious,