Long story short (quoted from above article): "A subcontractor that employed two of three construction workers killed Monday when scaffolding collapsed at a downtown Raleigh high-rise had been cited for four serious workplace safety violations in the last 10 years, according to federal records."
In the article dated 3/25/2015, the author points out that the General Contractor (GC) Choate Construction had an excellent safety record with OSHA, they sub-contracted out the scaffold erection to Associated Scaffolding which also has an excellent safety compliance record. However according to OSHA, Juba Aluminum Products Co. Inc., has a spotty safety compliance record.
Although the final inspection results from this tragic event have not been published to date, the question that is most prominent is; why did two companies, with excellent safety records, knowingly associate themselves with a company with a spotty safety record?
Prequalification of subs, may not have prevented this event, but is certainly would have raised safety compliance concerns and possibly increased oversight by the GC and Sub-Contractors. When subcontractor data are available (e.g., Suruda et al. showed that 59% of workers who died from trench collapses were subcontractor employees)
Other articles dealing with not prequalifying subs: