There was a broken door. The port authority was worried that people might use it to gain entry to the building and tamper with or steal some of the dangerous stuff they were storing in there. So they asked a crew to fix the door.

What would be the additional details? Welding is a pretty common activity in any sort of industrial area, especially when repairing large constructs made of metal. Sure, I guess there could be some salient detail we're missing, but welding alone is certainly a highly plausible catalyst.