Judging by the sheer amount of them with a few days time, I would definitely say they came inside the bag of substrate.
I run a sterile snake room, use good quarantine practices (and at the time I had no new snakes). I didn't bring in any new hides, plants, etc. None of my animals had been outside or in any position where they would have picked them up.
I know it's not a super common ordeal, but I am 100% certain I picked them up from a bag of forest floor. It wasn't a case of "Oh look there's a few mites in the water bowl"..........rather it was a case of "holy sh*t there's thousands of them all over the place".
Took me MONTHS to get rid of.