Why did Lady Catherine seem “almost angry” when she discovered that Charlotte and Liszt had seen a lot of her nephew Mr. Darcy?

One.... she was unaware of his visits, two, Lady Catherine had other plans for Darcy. Lady Catherine is a demanding and manipulative woman, she believes that she (and her machinations) should be put before everything else.

Miss Bingley’s designs on him were, by his behaviour to his cousin, for whom he was evidently destined by Lady Catherine, who talked of his coming with the greatest satisfaction, spoke of him in terms of the highest admiration, and seemed almost angry to find that he had already been frequently seen by Miss Lucas and herself.