I'm betting Rashta came to insist on getting a present and when she doesn't get one she'll use it as excuse to kick up a fuss. She'll say she came to tell the Empress she thinks of her as a sister but the Empress was cold and mean to her. Then the king being the absolute idiot he is will take her side immediately and further condemn the empress.
Rashta may play the idiot but I have a feeling she's quite cunning and has from the start been seeking to get the king wrapped around her finger. The king on the other hand is just being an emotional fool. Its true the empress may not be openly affectionate but I have a feeling he hasn't exactly put a lot of effort into the relationship. Not only is he forgetting all the empress does for him and the kingdom, but he's being easily swept up in a fantasy woman without thinking of the consequences. He wants a woman that constantly fawns over him, who is impressed by him, who is a delicate maiden that needs his strength and protection. A sensible, noble and capable woman like the Empress isn't going to play out that fantasy for him which is why Rashta is irritatingly brilliant in her acting. She's made his unpractical and self-indulgent fantasy come to life and he's eating it up.