The specific, supposed violations weren't mentioned in the article. It was just mentioned, as an aside, that Spotify claims Apple is pressuring them into forcing all iOS Spotify users through Apple's pricing scheme.
If the violation were that Apple was forcing Spotify to pay Apple 30% for subscriptions, then that would be a very clear breach of the spirit of anticompetitive laws in the United States, considering the existence of Apple Music, and considering that it would force Spotify to either raise its prices and reduce its consumer appeal or take a substantial, perhaps fatal, revenue cut.