fans keep running even when the computer is off

i just replaced my motherboard, the old one got damaged because thermal paste leaked onto the pins. So i replaced the mother board and made sure all the wires and other components are in the correct place. I turned on my computer and it worked fine. I decided to turn my computer off, and this happened. The monitor, keyboard and mouse turn off but the fans keep running and the only way to stop them is to press the power switch on the back side of the computer. What causes the fans to keep running even if the computer is switched off. (Its a desktop btw)