The condensation-induced atmospheric dynamics posits condensation rather than a temperature gradient to be the driver of winds (see here for a more detailed discussion). The conventional explanation holds it rains during the wet season because land is warmer than the ocean and the moist air comes from the ocean to ascend over land (because the warm air rises). When moist air rises, it rains. It does not rain during the dry season because the land is colder and the cold air does not ascend.
Let us now look at this excerpt discussing the causes of floods in Thailand:
Other than looking at precipitation, one way to determine the difference between the wet and dry season is to look at air and water temperatures relative to each other. The dry season is characterized by cooler temperatures over the land, and warmer temperatures over the ocean. GLOBE schools in this region could verify this by examining both the Max/Min/Current Air Temperature protocol as well as finding a data source that provides sea surface temperatures from ships or buoys. Because of these temperature differences, a gradient forms, and the winds will blow from the Northeast, bringing cooler air in from China.
In other words, one thinks it is dry because winds do not come from the warm ocean to the cold land. In the meantime,
In early 2011, an unusual cold wave hit the region, and temperatures were below average for a few days. This caused the normal temperature gradient seen in the dry season to be even more pronounced. While this doesn’t sound like something that would cause major flooding, more convection formed due to the different amounts of heating and caused this devastating flooding.
In other words, it became even colder over land, but now it suddenly rained heavily.
Food for thought.
See also “Why the heat?” for an opposite case, when the land became abnormally hot, but not a single raindrop fell down (the 2010 heat wave in Russia).