Just a guess. But it is possible that when the bridge was built in the ‘60s the shoulder was not designed to carry the full load of 80,000 pound trucks bumper-to-bumper.
Now we are here 40 years later. The steel has developed some cracks and the contactor working on the repairs shoves all of the traffic onto the one lane that was never designed to carry it. The steel begins to bend under the heavy load of the evening rush hour, and the failure cascades through the rest of the bridge.
As I say, just a guess.
And another site. This one from the USGS that shows the river conditions at the MS River at St Paul. Once again, I know nothing about what I’m reading, but be someone out there does. :)
http://waterdata.usgs.gov/nwis/nwisman/?site_no=05331000&agency_cd=USGS
I’m still searching for an official depth at that bridge.