Maybe he thinks your just letting his enemies die to quickly. They get old, remember that, they can be tortured for a thousand years...
No, ot: maybe if you drop somehow below a certain line of DarkLord-reputation, he just miffs you literally every thing you do, that's not explecitly worshipping him? The question then would be: How did your DarkLord's Rep go down so badly? Wanted to place your goblin butt in the Orthanc? Or have you been a bad goblin and ignored his tasks and invasions? Or is it really something strange?