Hi Steepmountain,

It might be that the plant was unable to transport food manufactured by ites leaves down to its root system. Which eventually cause the system to malfunction and leads to its death cycle, and because of this the upper portion of the plant will die.

In creating a successful bud graft this should match the cambium layers of the scion and rootstock. The cambium layer is the green tissue just under the bark that is the actively growing layer. This layer holds the plant's vascular system that transports nourishment throughout the plant. Once this layers of the scion and rootstock are united, they will grow as one plant.

Hope this help.
Asked me if you still have question.