The main cause of flooding in the Yellow River are the melting snows in its upper flow. It has to be said though that that is not the only cause that contributes to the flooding of this large river, because the precipitation patterns, and the geography also play a big role.
The Yellow River starts its flow in the eastern part of the Tibetan Plateau. This is an area that has very large amounts of snow. Once the snow starts to melt it brings in enormous amounts of water into the Yellow River, be it directly or through its tributaries. That enormous amount of water from the melting snows results in flooding as the river itself can not accumulate all of that water.
Also, this river passes through several gorges, and that results in the accumulation of water as it can not pass through freely, thus resulting in flooding. The rain can also have big influence, as the Yellow River has a very large watershed, so large amounts of rainfall are bringing in lot of water in it, the end result being flooding.