I agree with Jean-Michel, the most likely cause for this is to have the option checked "Move Free media to Pool". Once Media is 'Free', that is, the Protection is either None or Expired, the Media is moved to the appropriate Free Pool to be used by any Media Pool that needs media to avoid a Mount Request
You can turn off thisoption, but, sooner or later (unless you manually move media to the Free Pool), the Free Pool will empty, and you will be prone to getting Mount REquests
