attacked neighbour!

We adopted a 3 year old female from a friend of ours who couldn't keep her any more because of long working hours 14-16 a day. She is a wonderful loving little girl and gets along great with everyone. We have had her for 7 months now. Here is my problem. Last night she got out the door, which she does occasionally and comes back right away when I call her. but she didn't yesterday she ran over to where out neighbour was cutting his shrubs and started barking and biting him,I ran over and yelled NO and she ran home. We also have rabbits that hang around that area, which she watches from our window. Any Ideas why she did this and how can I fix it? I would greatly appreciate your help.