The headline is misleading. They are saying that it seems to have failed due to some kind of mysterious fatigue failure at a wood to steel structural interface. The investigators are struggling to work out how because the timber was damaged when it hit bottom of the river and then damaged further when it was hauled out for inspection. The overloading in the headline seems to refer to the ‘load shedding’ to other structural members that will have occurred after the joint (or joints) failed. (I’m not a structural engineer, but my profession is closely related.)