I would assume an unclean cage would be the biggest trigger for mites. Is it true that they form from poopies left around the cage too long?
My corn snake had mites within the first month after I purchased him from a reptile store (about 5 years ago). My guess is he had mites before I purchased him. I just cleaned the cage real well and gave my snake a little soak in the tub and they've never returned.