Im not sure but i think they come from fairly fast flowing rivers and when they sit perched like that they can just sit and wait for any food to flow by right in front of them. Just my thoughts.
my 28" achara catfish has always done it too...I call it sitting pretty (reminds me of my dogs patiently sitting at attention for food). Don't know why, I think it's cute tho as he's totally propped up on his front side fins. I've never noticed my achara x tsn doing it tho. he just rests on his big belly