This is not your fault, you did nothing wrong. Snake mites live anyplace there are snakes, so there is always a wild resevoir of them. The important thing is to treat with PAM right away. Spray the substrate of every cage, with the snake removed, allow to dry completely, before putting the snake back in. Follow the instructions on the bottle exactly and PAM is safe and effective.
To relive discomfort of the snake you can use dilute Betadine baths, to kill living mites. Some mites will survive by crawling to the head. You can use vegetable oil on a Q-tip to remove these stubborn mites. Do not treat the entire snake with oil.
A line of borax one inch wide around the outside base of every enclosure can provide a barrier to mites. They cannot spread past it. Keep in mind Borax is toxic and must never be put in a tank. Do not use this method if you have children or pets that could accidently ingest the stuff.
If you use tubs in a rack, a strip of double sided tape on the outside of the tank around the vent holes will also provide a barrier.
You are going to see the mite problem be dramatically reduced by the Pam, but you may need to re-treat at least once. Some tanks will clear immediately, some will have a lingering problem, so it is important to keep mites from spreading from contaminated tanks to clean tanks. Also you have mites in your house, they will live for quite a while and they can re-infect your snakes if you do not form a barrier.
David