Is this a new plant this year, or an established one? Particularly if it is established, and if you are in a region affected by the drought-like conditions that quite a bit of the U.S. has experienced, I'm not sure that I would worry about it. I am not an expert at too many things, and certainly not this, but we have a dozen or so burning bushes, which I really enjoy by the way, and they have always seemed to me to be very durable. We have one that was about 8' tall in 1988, when we also had a hot dry summer, during which we simply were not able to provide enough water to keep everything going, and that plant wilted literally to the ground, with no hope of ever being revived, but we had a sprout there the next spring, and it is presently about 10' tall and about as wide. This year, they are holding up much better, but we still have a few that appear to be getting ready for winter much, much earlier than normal. I would suspect that yours is simply going into dormancy, or whatever, as a matter of self preservation.