It's normal behavior to determine who will be the dominant pig. Guinea pigs have a social hierarchy. It's normal for them to have a hierarchy and they need to know where they stand in relation to other pigs in order to have normal social relationships with them. Lower-ranked pigs are just as happy at higher-ranked pigs (and some pigs have no desire at all to be the dominant pig- my current old-lady pig, Sundae, came to us as a very passive companion for our extremely dominant older sow, Linney. After Linney passed from old age, the much older and larger Sundae became a very happy subordinate to a baby pig half her size!) They will still be friends- in fact, they need to sort this out in order to be friends!