My hypothesis on that is people responding to others' body language to get the same snap-out-of-dissociation effect. The people closest to Batman would see him and then look around at others more to gauge their responses. Others further away wouldn't see Batman, but would notice the more-attentive-than-usual other passengers and be similarly more attentive to try to find out what's going on. They then would notice seemingly unrelated things, like the pregnant woman, and respond more than usual. The paper also says Batman entered from a different door, so a ripple effect of attentiveness could explain this effect without needing responders to directly see Batman.