Packing supporting the bridge failed at one support point, causing a second temporary support to fail. The bridge slipped to its final resting position at both points. As the steel warren truss structure fell, one of the precast concrete slabs forming the bridge's deck came loose and dropped onto railway lines below.So it wasn't scaffolding or part of the support that fell down. The slab in question was part of a thin lightwight deck, probably designed to protect the line from workers above until the final deck was poured on top.
The article mentions that until the bridge is finished they can't bring materials from the Silwood Triangle worksite to the northern extension by rail, which means this cock-up could have delayed the whole East London Line project.
[via District Dave]