There's also the loads are balanced on the pylon. When the collapse took out the bridge on one side of the pylon, the other side failed as well. You can see that in the video as the bridge on the right side of the pylon is no longer counterbalanced by the bridge on the left side, and collapses.
Imagine you're holding up a barbell with heavy weights at each end. If the weights on one end drop off, the bar will tilt strongly in the other direction, dropping the weights off on the other end.
Imagine you're holding up a barbell with heavy weights at each end. If the weights on one end drop off, the bar will tilt strongly in the other direction, dropping the weights off on the other end.