If it was widened without increased strength, then that is possible. But the engineers would never let it get to a low value (lower than 2.0). I still think it is much more likely that corrosion, fatigue, and/or construction work probably killed this bridge. The photos are bad right now but I'm curious how the collapse progressed and where it started. That is the most important question that needs to be answered before we can determine the cause. The high temperature (in the 90s) is only helpful in this discussion so far as it makes brittle failure very unlikely (though it would be unlikely in any case since the engineers designed the bridge to survive Minnesota winters).
From the pictures on TV it looked like it might have been an upper chord joint, right over the east support bent that failed. The main span appeared to have dropped intact, and broken on impact.