Why do reapportionment occur?

Only seats in the House of Representatives need to be reapportioned. The number of representatives each state is allowed is based on population. To see that each state has the appropriate number of representatives, the Constitution requires that a census be conducted every ten years to determine the population of each state. States with a significant change in population since the previous census will see their representation rise or fall accordingly. For example, based on the 2010 Census, it appears that California will gain one seat in the House of Representatives; Illinois will lose one seat. In recent decades, southern and western states have gained representatives because of the increase in population in those regions. States in the northeast and around the Great Lakes have seen decreases in their populations, thus a corresponding loss of representatives.