Why do Holden think adults are phonies?

For several reasons. One of them (and only one) is "herd mentality", such as when Holden is in the nightclub, and everyone is listening to "Ernie" play the piano. When he finishes, the crowd applauds wildly, and Holden observes "all the phonies went mad with joy". He also observes that Ernie bows "like he's this real humble guy or something". He describes the entire nighclub by saying "the place was crawling with phonies".