There was a lot of speculation earlier this month as to why Lady Gaga visited Apple headquarters:

Could it be because Lady Gaga will be making an appearance at this year's Apple music event, confirmed to be on September 1st just moments ago? Maybe, she will even be performing. Every year at these Apple music events, at the very end Steve Jobs likes to announce a performer. Last year was Norah Jones. This year could very well be Lady Gaga, and would explain why she mysteriously stopped by Apple headquarters in Cupertino, California.