collapse of the old society

Yes, water combined with earth is scary stuff. The ground not being as solid as one expects is a stuff of nightmares... And growing population numbers let people build houses in areas they shouldn't.
Be it flood zones, mudflood-prone mountain valleys or areas with unsuitable underground for settlement.
Combine this with this happening predominantly in poorer regions that don't have the financial and technological means to omit the causes or at least handle the effects and you get the shockingly high number of effected people mentioned in the report.
And this will only get worse as extreme weather conditions increase in the coming decades...