I'd say that it is from overly dry conditions. Mainly, a lack of tight hides where the animals squeeze into them (literally flatten out and squeeze), which would make reduced airflow, which would allow them to conserve moisture. That, or that in combination with too much airflow in the rest of the cage itself.
How to prevent it? Reduce airflow in the cage itself, by covering most, if not all, of the top with something solid, like wood, glass, plastic, or even aluminum foil. Make sure the hides provided are very tight. And lastly, I, myself, would put a deep layer of dirt in the cage to allow the animal to construct hides (burrows) of its own design.
Some people will disagree with what I said above about reduced airflow in the cage and dirt. All I can say is, the successful monitor keepers have been doing that for a very long time (Frank Retes for a couple decades, and he actually developed these methods with other reptiles first). I personally keep my adult acanthurus monitors (like uros, a desert species) in metal livestock troughs with a solid plexiglass lid, with a good foot of burrowable dirt. They are doing terrifically and laying clutches every month or so (no brumation periods here). In fact, I'm expecting to dig up a clutch from my adult female any day now. My juvenilles are only 4 1/2-5 months old, and already at a potential breeding size. And, shall I ever obtain a pair/group of uros, I plan on keeping them the same way.

Oh yeah, Robyn of Pro Exotics kept some ornates in this manner recently. The grew faster than pretty much any uro kept in 'standard' conditions, and were multiclutching without any brumation periods, which are supposedly 'necessary' to breed most reptiles (false).
Opps, I kinda got carried away. Sorry if that was a bit TMI. I guess I'm kind of bored.
Cheers,
Ryan
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Please excuse my childish-sounding user name. Unless I created a new account, I can't change it.