The only reason I can see this happening like this is if Reelz is actually paying Dish to carry it in HD, and they only want to do it every now and then...
Since no one wants to search for themselves, I took the time to do it. This was posted by Scott Greczkowski on 3/26/2012: Effective Wednesday, March 28, Dish Network will launch REELZ HD temporarily as part of HD 120 and higher & HD Max. This launch is in conjunction with the premier of REELZ’s first scripted series, True Justice, starting Friday March 30, 2012.