Reading between the lines...."chain reaction....nausea....nearby passengers...no 'abnormal' O2/CO2 levels found (by emergency responders) after landing...zero air quality deficiencies" - it sounds to me like an "uncontained stomach failure" filled part of the cabin with vomit fumes.
United Airlines flight bound for LAX diverted in Colorado after passengers become ill - LA Times
It is, of course, possible that some other fumes triggered the initial event.