There are several causes of flood like due to heavy rainfall, overflowing of the river, cloudburst, snowmelt due to global warming, landslide of the nearby mountain of the river , Catastrophic flood etc.
Now I am explaining few of these reasons.
Global warming is causing disastrously to put us on the verge of creative flood. Temperature rise on each ear raises the sea level due snowmelt of polar region and also that of mountains. These excess water raises the sea level that ultimately is affecting the rivers to flow over its banks. At this situation heavy rainfall is adding to turn the situation in rampant and is submerging the non-flood zone also