Could be your bios settings, some can be adjusted manually so that the cooler fan speeds up or down depending on the temps variation, it seems that somehow your settings are... inverted, its slowing down as it gets hotter... check that first often they are a curve graph with min and max, or manual input values depending on the temperature, other than that it could also be a software controlling the fans, also inverted, slowing down as it gets hotter... what a mess.
older systems or as default some motherboards/bios settings just keep the fan at full blast providing a constant current of voltage, that being said, if that were your case the fans would have to remain on a constant speed independant of the temperature.