That a great example of hydrolics. What happens is the bouncing of the hydrolics while in transit foams up the oil and it slowly starts to expand. The driver or who ever is supposed to secure it. So while the driver is driving on his merry way, the arm or boom or whatever slowly rises and wham! It looks to me that it hooked the bridge and the forward momentum of the thing forced the arm up and through the bridge, pretty cool. Now, try and think of what was going through the minds of the truck driver and all the people driving on the bridge at that very moment.