If you have two pairs of bunnies, A & B live together and C & D live together. If rabbit B can see rabbit C he will want to defend his territory so becomes aggressive. However he can't attack rabbit C because they are kept apart. So sometimes rabbit B will be aggressive towards rabbit A instead to make sure everyone knows he is the boss.
It can also happen with a human, if one of your rabbits is aggressive towards you they may take it out on another rabbit near to them. :?
That is why it is best if you keep multiple groups of rabbits that they can not see or sense each other.