That's hiding behavior. One of my girls prefers hiding under newspaper over everything. Reptiles when they hide the smaller the better. If they feel the top on their back they feel secure even though the space they can walk is their entire viv. That's why many don't bother with the commercial hides, they're too tall. It's kinda like putting tape on a cat's back (kinda mean I know). If you've never seen it before, it tricks their bodies into thinking they're in a tight space, and they'll crawl on their bellies.
You've already gotten several suggestions already, so I'd try those or just keep with what you got. It probably doesn't matter to them what it is. My girl will ONLY accept a newspaper to burrow under. I use tile now, so I just put a piece in there if I notice her scratching around.
