It depends...children often don't know or don't understand why they sometimes feel the way they do. It's possible that he/she is trying to hold something in, then when they get home, try to vent out their anger. Is your child a boy or a girl? Although that doesn't matter..but if there is any family problems, he/she may be trying to vent off some steam in a way they dont know how to control...possibly let us know more about what he/she does. And if there was any problems in the family.
Why IS your 6 year old so angry? You're the one most qualified to know. Have expectations changed? Is the child spoiled? Are there problems in the family? Have you moved? Is there a new sibling in the house? Has your family changed, even if you didn't feel it was a negative change? Are the child struggling in school? Does the child have the skills needed to interact with others of the same age group? Does the child understand how to cooperate, follow rules, get along? Are the rules conistently applied?
when u figure it out, let me know. my 5 yr old is the same way. i don't agree with new sibling or moving. we have done both, almost a yr ago and he is absolutely horrible. maybe the first few months i can see it, but after things have settled down and schedules are back in full force i think it is an excuse and a cop out kinda.
im looking into adhd, and or odd. good luck!