Burning bush leaves turning red early, not all at once and falling off......versus other burning bushes in the area
Early color changes can be a sign of stress.
Spider Mite infestation is one possible cause.
Heat stress can also cause leaf color change, this is often the case when the bush gets more heat from a planting near a sidewalk or driveway or is getting reflection from a building.
Mechanical damage from mowers or weed wackers could also stress your plant.