What do you think would make a block fail.
The “block” lost internal integrity. It deformed diagonally, towards the downstream direction.
You lost a diagonal brace in there somehow, could have been overload, could have been a shoe collapsed, some crumbling of the piers (though they appear to be intact in the imagery) bolts connecting diagonals to uprights, etc.
Look at the picture in post #1404.
From the top of the nearest pier, follow steel vertical a short distance till you reach a diagonal that runs towards the top of the steel truss on the other near side pier. It’s purpose is to keep the trusses vertical over the piers and parallel to each other, so the deck can’t sway sideways, away from bearing directly over the piers.
From the available imagery, this appears to be where and how the failure occured.
Deliberate act has to remain a possibility, but my best guess says construction accident. On many jobsites, you have licensed and highly experienced engineers designing the final result, and much lesser mathematically inclined supervising the design of falsework and order of re-construction. Even if this level of management gets it all perfect, you still have the stiffs who may or may not carry out orders perfectly every time.
Gravity is a cruel mistress who tolerates very little error.