What kind of bush is it? Was it recently planted? Did the bush wither all at
Fungus, rot, boring insects, ph imbalance (unless extreme), lack of proper aeration or nutrients, excess (or inadequate) water, heat would probably cause a nominally slow decline and probably affect bordering plants as well. This sucker looks like it just died all at once; and none of the neighboring plants look affected.
My guess is that, if it was recently transplanted, overly aggresive root pruning and improper planting. If not, chemical.
"Those who cannot remember the past are condemned to repeat it".