Because the wood brige can't assume all the weight and also there were some people very exciting and shaking the bridge, That made rope broken and there were two pairs of concrete columns at two brigeheads also broken because they couldn't assume all weight. Result in the bridge turn on right side, and all people were falling down into the river.
And there was a lucky guy said when he has already passed the bridge, he said a lot of people hold torchs and shaked the bridge, after that he heard scream and saw people falling down into the river.
Immediately, firemen went to save people life. But luckily, the river is not deep and rapid, most of them can save their life by themselves.