Sideways bend. Nothing appears vertically distorted. No gravity loads apparent.

Clear day, now sun is shining - at 2:00 pm, right? But it's Canada.
Seems like the sun heated up those three girders - which were either jammed in place (no room to expand) or were wedged in temporarily in the previous days and night of cold, and were not anchored properly to the skid plates/expansion plates to allow movement.

The others were properly installed, or were the proper length to begin with.

Yes, buckling due to restraint would seem to be the cause. I think we had a similar thread some time ago, a bridge under construction in possibly Missouri, but I can't find it.

With little to no bracing at the top flange, I wonder if the plate girders buckled under self-weight?

If it were self-weight, would they not have twisted almost flat, or at leaned over sideways. I don't see any vertical movement at all.

the structure guys have picked this up http://www.eng-tips.com/viewthread.cfm?qid=383069

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Single bridge girder buckled and rolled outside of Denver, Colorado several years ago. Happened within a few days of being erected. Killed a family driving by as to fell. It was inadequately braced was the official finding

The girders look straighter at the bottom than the top. With improper lateral support the girders may have sagged in the middle thus causing tension in the bottom and compression in the top. Thermal may have started the lateral bend but I would bet sag due to lack of lateral support is what we are looking at.

Lack of adequate bracing. The current plate-girder designs are flimsy & delicate, just like bar joists. Makes them VERY hard to ship from the fab shop to site. They are nice and stiff AFTER the concrete road deck is poured. Not all that stable with all the 'diaphragm' braces bolted in, but w/o the road deck. LRFD at its 'best'; tall, thin, and wobbly until the last element - concrete deck - is installed. Not a hundredweight of extra steel.