I collide because I get too locked on to my target.  
Most often I misjudge my opponent's maneuver.  There have been a few times I thought my opponent was turning away, when he was turning towards me.  I ended up with a head on shot, and collision.  
When it happens again a few minutes later, I figure I've been flying too long, I am tired, and it's time to quit for a while. 
