I have a theory. I don't like my theory, but there might be some fact in it.
I keep thinking about the RIR chickens and now the Coturnix and wonder if the same thing is starting to happen. The Rhode Island Reds never used to be aggressive. They were Dad's favorite back 60-70 years ago. Now they are aggressive, not all lines but mostly. Breeders forgot to breed for temperament as well as to the standard.
So when you pick your breeders are you looking at just color, or just size, etc. I know I have an aggressive female out there, but I choose to breed her anyway because of her size. Yet in my chickens I won't put up with such behavior, no matter how close they are to their standard. But these are my meat birds and once she went to a breeder pen she settled right down. So I figured, OK I'd keep her and I still have her. Her daughters are not like her, but I have made changes since raising her, too.
But if you are selling birds or eggs you might want to put temperament on the list when selecting your breeders.
30 years ago the Coturnix were not aggressive at all. But I was only familiar with the Pharoah size and wild color back then.
Now I am breeding for size only. That was the plan anyway. Everyone of my breeders have been selected for size. I don't hatch and set every female I get in a breeder pen. I select only the largest from each hatch for breeders. Its taken me 8 months to get what 45 breeder hens out of hundreds and hundreds of birds. Now my standard is rising for my birds so there will be even less going into breeder pens. Should I take temperament into consideration, when I am after size and color?
But now I have Whites and they are the 1st generation from sports of the Big Jumbo Browns. So now I am trying to decide, do I select for their color spots (cause some are really pretty) but they tend to be a little smaller? Or do I select for size only and should I even consider temperament? Do I go for the pure White and set aside all else? Because they absolutely shimmer in the daylight and are just breath taking. These Whites are more flighty than the Browns but are much nicer in temperament so with these birds I don't think I have a problem.
But I can see where things might have gone wrong for others. Like the RIR and the BW they could get even worse temperaments if we aren't careful.