The causes vary around the world, but we can make some generalizations. Seasonal floods mean that there's more water coming in than going out for only part of the year. If it's annual, the changes are probably related to annual weather patterns in precipitation, temperature, or both. Some big sub-cases are:
(a) in Mediterranean climates, e.g. California, almost all the rain falls in winter in cool weather when evapotranspiration is relatively low. The Central Valley has regions that develop standing water from the rain that falls directly on them because the soil has impermeable clay or hardpan (the vernal pools prairies); and other regions that develop standing water because they're the confluence of several drainages and in wet years the water literally can't flow out to the sea as fast as it comes in (the Yolo Bypass).
(b) If annual precipitation includes a lot of snow and it melts in a hurry, it's common for the downstream rivers to flood at the confluences. There may be years in which the surface snow is melted while the ground is still impermeably frozen, which would turn anyplace level swampy. Parts of Minnesota have bedrock not far under the soil surface and they get this kind of inundation relatively easily.
(c) Where rivers meet the ocean in a long, nearly level bench, high tide (spring tide or storm tide especially) can raise the river level enough that the fields flood. I believe this is the case in the Somerset Levels and in the Netherlands. Some of these are watermeadows and can be quite productive if farmed carefully.
These don't all develop the same kinds of vegetation. For instance, I think I've forgotten a tropical or semitropical case where monsoon rains overwhelm the drainage (possibly the entire Amazon, where the hardwoods are adapted to having their roots underwater even though they aren't flooded all year).