Something is scaring them. What, I don't know. A shadow moving quickly across a part of the tank maybe?
Do they have lots of vertical cover? Tall plants like "Jungle Val" (Valisnaria americana) can provide a real comfort to discus, in my experience. (I've never like the bare discus tank approach.)
Chair, American Cichlid Association (ACA)
President, Oklahoma Aquarium Association (OKAA)
ACA 2012 Convention in Indianapolis, INhttp://www.aca2012indy.com