However I imagine it has something to do with how callously Adar is throwing away the lives of his "children" at the walls of Eregion. The second in command Orc (Glûg I think?) seems very disturbed at how Adar would rather all the orcs be killed than serve Sauron again. Being a master manipulator I imagine Sauron uses that feeling to sway the Orcs to his side. As an aside I know Adar isn't canon but I think having one of the first Orcs who were twisted elves who followed Morgoth as a character is really cool.