1) In a surprise turn of events, the Dunedain catch up with the Nazgul and, outnumbering one (not the Witch King), manage to banish it. Nagul can be banished by exhausting their Will points or by causing them one wound. Being quite tough, a 6 is required for the Dunedain to wound a Ringwraith. This might not seem so hard when in this instance there were 3 of them rolling to wound, but remember that the Nazgul in this scenario are quite weak and have no Might or Fate points. In other scenarios they are extremely formidable.
2) Another shock! Both the Dunedain who have been attracted by the noise of battle were clearly not expecting to stumble across a Ringwraith and fail their courage checks to charge.
3) The lone Nazgul seems to be doing a good job of breaking through and kills one of his pursuers.