When a mature shrub dies quickly but the rest are fine I suspect a root problem. Most likely a mole, vole or gopher has burrowed in. The best way to confirm the diagnosis is to dig up the brown shrub and examine the roots. You should also look for signs of burrowing. A less likely cause could be fast growing root rot from poor drainage - such as if a sewer or water main is leaking and this shrub is a low spot where the water collects. Note poor drainage when you dig into the rootzone of the shrub as well. Keep growing, Marianne Binetti