According to a Transport for London spokesman: "The bridge was moved into position earlier in May, and the contractors Balfour Beatty and Carillion were on-site to carry out some further work.
"The bridge was held on a temporary jacks, some 8 inches above the bridge's final resting point.
"One jack gave way, which led to the bridge move suddenly to its final position. This caused some panelling to fall from the bridge to the tracks below."
The spokesperson said he believed that the bridge's final move had not been planned to take place yesterday, and some concrete shuttering was being erected, with preparation works for the final move underway.
The spokesperson could not confirm whether the jack had malfunctioned, or had been shunted from its position, or whether another structural malfunction could have caused the sudden move.
"Network Rail, Transport for London, Balfour Beatty and Carillion were on-site to ensure safety," said the spokesperson.
Network Rail confirmed that limited services re-started around 9.30 this morning, with a full service expected to resume by lunchtime today.
At 7:25pm yesterday, the driver of the 1915 service to Southend hit debris in the vicinity of GE19. Power to the lines was turned off as a safety precaution.
Officials from the Rail Accident Investigation Branch were at the scene.
The 1,300 tonne bridge had been inched across existing railway tracks using hydraulic rams, as part of the East London Line extension into Shoreditch. Main contractor is a Balfour Beatty/Carillion joint venture.