I've gone through this game before, just picking it up for a refresher.
In the very last room, if I don't go in and shoot the enemies from the tunnel, there is usually a long period when no one comes out unless I go inside. What makes them come out? Just wait long enough? I think I had instances where I waited in the tunnel until the machines guns open fire, but this time, no matter how long I wait, the last group of guys before the machine gun just don't see to come out.