Allegedly, that particular support post was the key one. Only people familiar with the design would have known to hit that one in order to do maximum damage.

Also, if you watched the video of the boat in slow speed, you see it purposely turns hard into the support post. And as the bridge breaks into 3 pieces and collapses, you can see at the 3 break points on the top, a flash of bright light occurs (explosion?) right as the bridge snaps at that point.

So, there may be a lot more to this.