while i dont know anyone who failed in the interview (my acquaintances mostly failed in the written ) i think we can safely speculate about the possible causes..

#1. insufficient prep. (either those candidates were busy with something else OR they werent expecting to clear the written. either way weak preparation probably brought about the failure. imagine not being able to answer 10-12 questions in a row!

#2 Abusive attitude (sounds surprising right? but there ARE such people who fly off the handle on certain questions. once a Baloch friend of mine admitted to losing his temper (on a question related to insurgency in his province) & using abusive language with the panel (while he didnt fail, he barely passed the interview)

#3 Extremely negative psychological report (*sigh* many people believe that psychological report is not that significant. perhaps for normal (psychologically sound) candidates it might hold true.. But if the psychologists diagnose someone with serious nervous system disorder(s), the panel might feel it safe to fail the candidate to prevent his allocation.