Broad generalisation there I know, and I am not accusing your child of being a bully if they cried when they started school, but I have often noticed that the children who don't want to separate from their mother and cry a lot, go on to be, shall we say, less mindful of other people's feelings.
Why is that? I mean, from a psychological POV. I should know as I studied psychology as part of my teaching degree, but I just can't think what the reason would be.