It could be any number of things including...

-Boredom
-Striving for independence
-Bottling up emotions and then lashing them out in other ways
-Testing her boundaries with you as a parent

And the truth is you may never know what it is if she is not able to verbalize why she is acting out the way at she is.  The best way to gather information about what she acts that way is to try and catch her at a time when she is content and doing well.  At that time ask her if everything has been okay lately.  Ask her why she does certain things or acts out in certain ways.  Do so in a nonjudgmental and open way and see what kind of response you can get.  You could even probe her by saying things like, "I know I get really mad when..." and then presenting her with different possibilities from your perspective.  This may be able to help her verbalize her feelings without feeling like she is getting in trouble.  

Also continue to praise her when she is behaving well.  Tell her you are proud of the way she behaves at school.  Tell her you are happy to hear when she does well at tasks.  Sometimes we only catch kids being bad, but it's great to catch them being good and providing positive reinforcement.  

If you continue to see these behaviors and become worried about her well being, it can't hurt to get her to a child psychologist for an assessment.  Sometimes the answers they provide about behavior can be extremely insightful and the sooner you figure out what exactly is going on, the sooner you can address those problems.  

Please take care and remember that you can call the Boys Town National Hotline 24 hours a day at the number below, free of charge to discuss any parenting issue.  We are here to help, you are not alone!

Counselor, NZ
Boys Town National Hotline
www.parenting.org
1-800-448-3000