Exit doors open outward so it would have been dang near impossible to kick in. It could have been opened by someone inside the theatre. The perp could have gone to an earlier showing and left the door wedged open slightly. An explosive charge is a possibility - in multiplex theatres, I frequently hear booms and blasts from films in the adjacent theatre - I likely wouldn’t have been concerned if the noise wasn’t extremely loud.
You are making this much too complicated: People are sitting in a movie theater having a good time, expecting nothing bad to happen. Someone starts banging on the exit door from the other side. Maybe he's yelling "I locked myself out" I'm willing to bet that 8 out of 10 people will open the door for the person if only to get him to stop banging. The person who opened the door might have even been an usher (do movie theaters still have ushers?).