Best guess for why the bridges failed, in the absence of any real information, is corrosion. Bridges like that are built from prefabricated sections of prestressed concrete, which means ridged steel rods called rebar are embedded into the wet concrete and as it cures it puts tension on the rods. It's clear from the news photos that a whole section fell onto the roadway below, which suggests it wasn't the concrete that failed, it was the steel clips that hold the sections together. Those clips are normally what engineers describe as quite ductile, which means they can take quite a lot of stretching and deformation without failing. They have to be, because the sections will expand and contract, and the whole bridge will change shape slightly, with temperature changes. Corrosion, possibly due to roadway salting in winter and improperly maintained surface sealing, makes the steel lose its ductility, it'll become brittle, and fail suddenly and completely, instead of deforming elastically. Corrosion's not always easy to spot either. It won't necessarily be as obvious as surface rust, it could be eating away at the hidden surfaces of the clips while the visible parts look perfectly alright.
The recent failure wasn't an old bridge. The expected lifetime of a bridge like that is about 75 years; that bridge was about half that age, according to what I've heard on the news today.