Snake mites are natural ectoparasites of snakes, and are usually brought in via new arrivals that contained mites and/or their eggs. The most common scenario is for a new animal to arrive with mite eggs, which are nearly invisible to the naked eye. A number of days later, the snake suddenly begins to show the typical behavior--with prolonged soaking being the most obvious sign. If you don't catch the problem early, then your entire collection can be affected, at which point it can turn into a bit of a nightmare. You can largely prevent this situation by subjecting every new arrival to a prolonged quarantine period (six to eight weeks is usually sufficient for mites and other stuff, but generally the longer, the better) in a separate room.
There is also the possibility of mites hitching a ride via some sort of vector, such as feeder rodents from a pet store. This is far less common, but it does happen.
Here is a decent article by Dave & Tracy Barker that gives you a pretty solid introduction to the life history of snake mites.
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