The UC Berkeley-Stanford study found that all children who attended preschool at least 15 hours a week displayed more negative social behaviors such as trouble cooperating or acting up, when compared with their peers. The discrepancies were most pronounced among children from higher-income families.
The kids are getting more educationally, but they are also under more pressure - so they compensate by acting out in that typical toddler way. (my 2 cents)
I don't know that you can draw that conclusion from that correlation. It could say as much about the environment specific to those pre-schools as it could about whether it was specfcally academic preassure. It could be that away from mom and dad they are in a less disiplinary environment. Also, what happens when you compare them with the kids a grade ahead. Do those kids naturally exhibit similar behavior? It could simply be early onset of natural child development.