how our kids act is often a mirror image of how we act, perhaps she is seeing you as being a bully for putting her in T/O regularly. so she sees when she is a=in a positon with others the way to get what she wants is to be aggressive..
perhaps looking at other ways of dealing with her tantrums at home will help her to deal with situations out of the house..

I know my children often mirror my less desirable personality traits