I had a rose of sharon for many years that got to 8′ tall in 2016. This was the first year I noticed it didn’t fill out complete
ly. Last year in 2017 only about 1/3 of the bush leafed out and this year nothing. I noticed that part of the base was rotted but no idea what caused it. Was unsure about replanting the spot with another rose of Sharon.
A shrub can die off for many and various reasons.
Winter weather can be an issue, heavy rains or drought. Soil that doesn't drain well can cause root rot. Pests or disease can cause the shrub to decline and die. A shrub also can die for reasons that are never discovered.