Dollars to donuts the vast majority of these kids come from single parent homes.
So many of the comments on this thread are on the right track.
First of all, the Campfire center featured in the article is the best childcare facility in Fort Worth. In fact, they offer training to other centers, teachers, and home care providers. However, it is a daycare facility, and not what most would consider as a traditional pre-school. This family found a good place for their child, but it is still a daycare center.
Secondly, the lack of discipline in the home is readily apparent in any classroom, but especially so in a full day program for preschoolers. What the article doesn't mention is that these aggressive children often harm other kids through fighting, biting, kicking, rough play, etc. If a child is expelled, one should suspect that other parents may have complained about those types of behaviors directed at their children.
Unfortunately, if this aggression isn't addressed at an early age, wait and see what they do in first grade.
Taking many years off to raise my own children in the home was the best sacrifice we ever made.