The overdrive channel of an amp compresses the signal, which makes signals with greater amplitude less loud relative to the original signal, and softer signals more loud. Thus, you hear more hum and noise.
Humbuckers were designed to prevent hum, hence the name. Split them, as others have said, and you get the hum single coil guitars have. It's all about the pickups amplifying electromagnetic interference.
This can be caused by lots of things, like appliances, light dimmers, wall warts, cables with poor noise rejection, electrical wiring boxes, computer screens, amp transformers, and goodness knows what else. Not only does facing in a different direction often kill the hum, standing in a different spot in the room can solve it. If you use pedals, ground loops can also hum and make normal hums and buzzes worse, so working on eliminating ground loops and other sources of noise is a good idea.
Or simply don't split the coils when using the overdrive channel.