That is tructural faliure. The entire slab fell down as it was not fastened properly. If you blame on truck driver, then the real culprit (construction company) will slip away. I don't think the weight of the truck exceeded the bridge loading (by seeing the picture.) As higher load should buckle the bridge whereas it fell on its side.
Anyway, feel sad for the hardworking people who lost their lives.
@aziqbal
If people fail to understand you so then you should just keep posting your opinion, even if you are balmed and accused. Why to rift away from your conutrymen. Answering (1-2) is better than -ve ratings.