Yeah that's a tendency quite a few professionals have with mass casualty situations (I've fallen victim to that myself), once they start treating one patient they'll get "hung up" on that patient. 
  I think that is primarily due to having it drilled into us that once you establish contact with a victim/patient leaving them is abandonment.  In a MCI, aside from the emotional enormity of encountering such an incident, it can take great willpower to override that sense of duty to a particular individual and move on to the next victim/patient.
Pete