Ethan Shanfeld When Fred Again casually mentioned to his 2.2 million Instagram followers on Tuesday that he was in New York, his most ardent fans already began planning their pilgrimages to the Big Apple. No shows or appearances had been announced, but the dance music darling’s history of pulling surprises and a scattering of subtle hints was proof enough that something was afoot. And when, after a series of teases, Fred formally announced a pop-up show at Knockdown Center just hours before it would begin, he sold out 3,200-some tickets in less than 10 minutes.