I don’t think it was terrorism, even though the absence of evidence of terrorism is not evidence of the absence of terrorism.
The main focus of many terrorism attacks is to affect the economy of the West, and to cause the greatest economic damage possible. A bridge of this importance would fit the criteria, and may be a viable option beause the more highly important targets are being guarded more closely.
IMHO, it was a combination of structural defects that will take a while to doucment. I think the prime suspects are the undermining of the South Pier from water flow through the nearby lock (shown be pictures on this thread from normsrevenge, combined with a weakening and excessive expansion of the steel from the removal of the concrete on the deck. This exposed the steel to increased heat from direct sunlight (otheriwise the deck shades the steel), and this caused the steel to expand more than normal in the hot weather.
We are all entitled to our groundless speculation, aye?
Someone last night mentioned that a nearby dam failed about fifteen years ago. This could have caused weakening of the piles that wouldn’t have been a problem UNTIL the expansion joint was weakened. When a critical load is damaged, portions that were not previously critical can become critical.