It's not weird at all, the electricians had a picket a month before because they weren't even using licensed sparkys, let alone union. When you hire the cheapest unqualified people to do stuff, this is the result. I would guess either faulty welds on the structural steel or improper/ partial bolt up of the steel. When assembling structural steel they usually only put a couple bolts in to hold it in place, then a bolt up crew follows behind a few floors down. If they don't (are behind schedule, etc) this could happen as well.