The area has always been foggy. Mainly because it gives Giant Chasm a creepy effect. But, on rare occasions the Giant Chasm isn't foggy and there will also be no fog in Giant Chasm on your birthday.
There are 2 reasons (one is science related) and the second one is actually Scizornicians answer. This is an icy area but heat will pass through forming water vapor which is pretty much when built up creates fog. Also scizornithians creepy effect is probably the more probable answer