because there is to much water.
Ok, a more detailed geographer explanation:
Places flood of course because there is indeed to much water. In this case it was rain. At one point during the rainfall the soil cannot take up any more water as it is basically saturated, this leads to surface runoff; in todays case with barely any free soils but loads of sealed surfaces (roads, houses etc) the runoff starts immediatelly. This surface runoff should be collected in drainages; however, this tend to get blocked or are not build to deal with that much water.