More often than a faulty initial design, it's because of a something not being followed, e.g. the design called for one type of material or process and another was used during construction, either on accident or on purpose but without correctly doing the math to verify that it will work.

This makes a case for engineering margins, maybe even running the numbers assuming a worse grade of steel or bolts than specified. Also worth remembering this building wasn't special. If this was a design or construction flaw that surfaced with added load, a lot of other buildings from that era probably have a similar issue.