An unexpected reaction is not, by itself, a violation. What an investigator looks for afterward is whether the written programs that are supposed to anticipate it existed and were followed. Which programs those are depends on how much chemical was involved and how it was being used.
If it was a covered process: PSM. The Process Safety Management standard applies to a process involving a chemical at or above the threshold quantity in its Appendix A (29 CFR 1910.119(a)(1)(i)). For a covered process the employer must perform a process hazard analysis appropriate to the complexity of the process that identifies, evaluates, and controls the hazards (29 CFR 1910.119(e)(1)), must have written operating procedures with clear instructions for safely conducting each activity (29 CFR 1910.119(f)(1)), and must train each employee in the process overview and procedures, with emphasis on specific hazards and emergency operations (29 CFR 1910.119(g)(1)). After the event, the employer must investigate each incident that resulted in, or could reasonably have resulted in, a catastrophic release (29 CFR 1910.119(m)(1)). A test run on a schedule that skipped the analysis is the detail an investigation will find first.
If it was laboratory-scale: the Lab standard. Employers engaged in the laboratory use of hazardous chemicals fall under 29 CFR 1910.1450 (29 CFR 1910.1450(a)(1)), which turns on the standard's definition of laboratory use: small quantities, non-production, multiple procedures, and protective practices in place (29 CFR 1910.1450(b)). Those employers must develop and carry out a written Chemical Hygiene Plan (29 CFR 1910.1450(e)(1)). A novel sample with unknown reactivity is precisely what the plan's provisions for prior approval and special precautions exist for.
Getting people out: the emergency action plan. An emergency action plan must include, at a minimum, procedures for reporting a fire or other emergency and emergency evacuation procedures including exit route assignments, along with procedures to account for all employees after evacuation (29 CFR 1910.38(c)). It has to be backed by an employee alarm system with a distinctive signal (29 CFR 1910.38(d)). An evacuation that worked by word of mouth is evidence the plan was not doing its job.
If anyone went toward the release: HAZWOPER. Employees who respond to an uncontrolled release from outside the immediate area, rather than evacuating, are engaged in emergency response (29 CFR 1910.120(a)(3)), which requires a written emergency response plan, trained responders, and an incident command structure (29 CFR 1910.120(q)(1)). Employers who evacuate everyone and let no employee assist are exempt from that paragraph, which is a strong argument for making evacuation the plan.
Protective equipment sits underneath all of this, not in place of it. PPE is selected after a documented hazard assessment (29 CFR 1910.132(d)), and a good suit does not substitute for a hazard analysis that was never done.