Every place is different, but in areas with tight clays it is a real problem. People will plant the trees using a post hole digger. This causes shear sides. The roots will go to the area of least resistance and continue growing in a circle. A root can also go in a circular pattern and as it gets bigger it can chock out the other roots. It doesn't always happen but does pretty often.

I use to argue that potted plants work better but after planting several trees the bare root trees always seem to do better as long as they are planted early and watered when needed.