I'll explain again for people to understand relative motion between two moving objects.
If two airplanes are moving towards each other, they will move in the windscreen. If you are looking at a plane coming towards you from the left, - movement from left towards the right indicates that you are lagging them and will pass behind them. If they are moving right to left, it means you have lead pursuit and will pass in front of them.
If you are on a converging flight path they will stay stationary and get bigger, which what happened last night. Here's the kicker, at night you don't see the airplane getting bigger because you can only see the lights, and at night lights can cause some interesting Visual illusions. Also, you have to pick up the light from amongst all of the other lights that are on the ground. And at that altitude, you would definitely mix in with lights on the horizon. On top of that your night vision can cause stationary objects to vanish. Try it sometime, go outside and stare at a distant light, it can disappear in the center of your vision. That's how you can fail to see an airplane.
Now before the CRJ made their final turn, they would have have been moving from right to left in the windscreen, possibly making the helo think they were going to pass behind because the CRJ was approaching from their right. Once the turn was made, they became stationary and were now on a convergence, but the landing light was aimed at the runway, not the helicopter coming from the right and they may have lost the aircraft then.
