for compression, and any remaining capacity of the existing columns would be the first thing engineers, building department etc. checked & verified when designing the addition. Being steel it's possible they were too tall and/or slender for the new loads, but this is still SE101 stuff and virtually impossible to get through all the checkpoints without being flagged and octuple checked. I saw a photo in another article (can't find now) that showed a sagging beam crushing a steel stud wall below. If that was a beam spanning those failing columns then it would suggest new / excessive loading midspan on the beam. That would then add new eccentric / lateral loads on the columns which could certainly cause the bending and twisting seen in the photos. So either a faulty detail or field execution that is causing load to be applied midway on beams instead of centered on columns. It's hard to piece together with the scattered info out there so far, but that would be my guess based on what I've seen / read. Only certainty as of now: many rich lawyers about to get richer.