If it was a grafted tree,the pink blossom will be flowering on the root stock.

If it's supposed to be pink, the white flowers are probably the rootstock. You probably had a sucker come out below the graft, and grow vigorously up through the crown. Another, less likely, possibility is that there was a mutation (called a "bud sport") near the top, which produced a white-flowering branch. Doing the "child's puzzle", and tracing the branches, will answer this question more definitely.