The deck can act as a membrane for lateral stability, but that adds greatly to the dead load. In mechanical effect, you have a zigzag series of braces to transfer the lateral loads doing it that way, whereas a very few diagonals connecting the cantilever truss systems accomplish the same objective.
This is what the pictures tell me:
The problem began on the right side of that picture.
Pier footing, pier or steelwork.
Bank on it.
I don’t say that very often, but pictures don’t lie.