Jeffers, I am intrigued by your theory on this.
It disturbs me that that the pier on the south(?) side is so out of plumb, post collapse. The north side pier has both towers intact, except for a violent reaction at the top of the piers, where the rocker plates have been ripped off the top of the piers.
The other pier has both rocker plates intact, and the span appears to have snapped cleanly off at the pins.
If something happened at the north side, with a lateral deflection sideways of the span(not the piers), I could see the center span pulling at the south pier, pulling it toward the center of the river. The half arch span to the south broke cleanly off once the southern pier went out of plumb, and leans, almost intact to the south.
But what if there was scour at the south pier, which is at the waterline and has no lateral support similar to the support enjoyed at the northern pier. What if that pier went out of plumb and twisted toward the river, making both spans it supported rip off their rocker pins, and the center span falling into river, tore the northern part of the span and twisted it off of the northern pier, which is not out of plumb post collapse.
The fact that the southern pier is now out of plumb is disturbing, because it shows that, in either case, it was the weaker of the two piers.
The slide show posted by Norms Revenge earlier shows some more shots of the southern pier.
Although the rocker plates were ripped free of the pier by lateral deflection, the end placement of the kingposts indicates they rotated about their bases, displacing the tops of the piers little if at all.
On the other side, you could be right, but the tension in the cantilever trusses would come as a result of the lateral deflection, not an increase in pier to pier distance as the piers at the failure point didn’t move.
A good strong jerk along the cantilever steelwork could begin the pier deflection process, which could continue as the sidespan adjacent to it fell and kept shoving them towards the river.
Not real impressed with those piers, I don’t see much more concrete than necessary for vertical load bearing, not a lot of lateral stability there, they appear to leave that to the steelwork above.
Still, it’s clear that the triggering event initiated on the far side.