"I want to branch out and gain new experiences to broaden my skillset." is the BS line I usually go with when I'm interviewing for a job outside my current specialty. Which has been every job I've interviewed for. Since my organization has rotation programs, it's a well-accepted reason. Especially when the actual reason was "I'm fleeing a hostile takeover of my last department." or "I gotta get the fuck out of Florida, it's 2017."