This typically happens when they are installing an unvented crawlspace in accordance with R408.3.  They typically do not have the insulation, vapor barrier and the dehumidification equipment installed until the end of the project, which allows for moisture to form during the rough/framing stage.  Fresh pea gravel usually contains quite a bit of moisture as well.

Personally, I have required mold remediation to be performed before passing the final inspection.  Although, I have heard multiple times that; It's not mold!  Its only Mildew! :)