Well i dunno , i doubt this would be a terrorist atttack . Terrorist would pick a much bigger target than a bridge to cause much more death and destruction
Also they would have planted there explosives re opened the bridge and waited for all lanes to be full then blow it up .
Mind you in todays society you can never say never but i think if there was construction work being carried out then that could have caused damage .
Nobody thinks anything of vibrations but near some of my relatives house house there was a new bridge being built and the vibrations from that caused cracks to appear in walls so i think if they where drilling the vibrations could have caused further damage to the current damage that was there or could have caused some kind of damage to the structure