I believe that the reason for adopting neighbouring fry is to promote the safety in numbers benefit for your fry. In a 50/50 situation were equal numbers of biological and adopted are present, the benefit only comes if the adopted are smaller than your own, due to the smaller ones being easier targets, and maybe even providing a meal for your own young.
With only six youngsters left the parents "decided" that the cost of defending such a small number didn't increase their chances of survival enough for it being worth the effort. With the arrival of the small fry the safety in number pushed the defending effort onto the beneficial side once again. Maybe because the six that remained will have a skyhigh survivalrate compared to the new-comers.
The thesis may be a bit forced, but the the situation is very strange, and calls for unusual theories.
Another question is why the other pair allows for their young to switch sides, when it leads to death and destruction. Both on the way past the amarillos I suppose, but certainly upon arrival.
What sort of behavior does the right pair show when their youngsters depart?
Jacob (who is very gratefull that you share your river/lakeside with us!)