My burning bush is being stripped of a third of its leaves and I found another one that has lots all leaves on a couple branches. The lost leaves go from bottom to top.
Burning bushes are quite robust to the point that they are considered invasive. It is possible that, with out wet summer, they have picked up a fungal disease. If that ids the case, they will bounce right back next spring. Another possibility is that bark mulch has bee piled up against the bark of the bush…this slowly strangles the plant. Get down there and brush the mulch away from the trunk of the bush until you find the original soil level and keep it that way.