It is certainly possible that root rot is occurring and not (yet) affecting the others. Drainage can be surprisingly variable sometimes from spot to spot in the yard, and it's also possible the plants came with a pathogen contaminating the potting mix and it's only impacting the individual plants that were somehow stressed by something in the environment. 

Unfortunately there is no practical treatment if it is root rot, since the very few preventative soil drench fungicides don't work reliably and cost a lot more than other fungicides. You can just trim out dead stems and see if the plants replace that growth. 

Thielaviopsis , "black root rot," is most likely the cause since Inkberry are known to be one of the vulnerable host plants for this fungus. Despite the species' tolerance for wet soils, in our experience Inkberries in home gardens tend to contract this (or die from whatever is causing these symptoms) regularly. This might be due to a greater vulnerability of certain cultivars or maybe a factor that's stressing them that horticulture hasn't identified yet; 

Thielaviopsis doesn't do well in acidic soils, so maybe a soil test would help determine if acidifying the soil would help the Inkberry roots thrive. 

Hopefully these changes can help the inflicted plants to recover. 

Emily