“Backrooms” is part of a growing wave of breakout films from fledgling directors who honed their instincts on YouTube rather than inside the Hollywood ecosystem. Two other creators with no Hollywood track record — Curry Barker and Mark Fischbach — have already turned online followings into surprise box-office hits this year. “It’s not an anomaly,” Stephen Galloway, the dean of Chapman University’s film school, said in a phone interview. “It’s the start of a gigantic shift. These are the cinematic insurgents of our era.”