We don't have Darcy's own explanation for his behavior yet, so we can't discuss that, but we do have the narrator's explanation. The narrator tells us that Darcy is haughty, not that he appeared haughty.

Darcy doesn't say he is shy at the piano at Rosings. He basically says he doesn't like small talk, and that he can't appear interested in other people's concerns. His cousin, who knows him well, says it is because he will not give himself the trouble, not because he is shy. Also, Darcy makes a series of rather transparent excuses. He hadn't been introduced to any lady at the Assembly? I mean, really.