Ojiugo is Okonkwo's third wife and also the youngest. As it is in a patriarchal society, the wives have to respect their husbands and follow their instructions. It is also customary that the youngest (or last) wife stands at the bottom of the hierarchy. She is supposed to be dutiful and respectful towards the elder (or first) wives and has to honour their authority - there is, of course, mutual respect between the wives, although disagreements might occur from time to time. In such instances, the intervention of the elder wife is needed to settle a dispute, before the husband is consulted, if at all.
Ojiugo, being the most junior spouse, therefore has a lower rank than the elder wives. Because of her youth, she will be more inclined to neglect her duties since she is 'still learning the ropes', as it were, although she must have received guidance from her own mother, as well as the elder wives, about the duties of a good wife. It is this which leads her to have her hair braided during The Week of Peace instead of performing her wifely tasks, such as cooking. During this week, calm descends on the village and men relax, leaving their wives to complete their domestic chores.
It is forbidden for anyone in the village to indulge in any violent acts during this period, but Okonkwo, enraged by Ojiugo's actions, strikes her. This act is extremely disrespectful within the context of their culture, specifically at this time, and is one of the reasons that Okonkwo is later banished to his mother's village, Mbanta.