I think it's what others may call sulking.
They do that from time to time, usually when something changes in their surroundings.
I've seen loads of situations, that triggered that behaviour.
Here's mine, just laying on the gravel sideways, resting against a stone.
as long as they show interest in you and food, it's nothing to worry about.
I even saw my red freak at some visitors and lay on the gravel, without moving anymore.
They respond totally different to different people.
edit: my oscars seem to love the current in my tank and are often inspecting the inlet and outlet for food.