I thought it was a combination of bird shit buildup and the gusset plates already being to the wrong thickness. The acid in the bird shit caused the thin plated to deteriorate faster
I know about the gusset issue.
Not up on my bird poop science.
If you can believe the Weather Channel aired last night a one hour documentary on that collapse. It was blamed on construction at the time. And a delivery of materials piled in one vulnerable part of the structure, where the gussets plates were 1/2 the design thickness. Also of note were the lack of movement in rollers designed to carry freezing shifts in the structure.