The thing is, dominant white can't just pop up out of nowhere. It's not a recessive gene. Good point about the egg color, but Delawares are also supposed to lay brown eggs. Are you sure that none of your EEs could have laid that egg?
Out of curiosity, what colors did you get? You said in your other post that some of the red sex link mixes were black. The fact that most of your chicks aren't black isn't that strange. It's usually assumed that black crosses will generally be all black or mostly black because it's a pretty dominant color. However, buff x black can produce mostly buff-colored offspring, red x black can produce black with a lot of red leakage, dominant white x black produces white chicks, Columbian x black can sometimes even make fully columbian chicks, etc.