Final answer:
Changes in exports and imports occur primarily because of relative price differentials among nations.
Step-by-step explanation:
The occurrence primarily caused by relative price differentials among nations is changes in exports and imports. When goods in one country become relatively cheaper compared to goods in other countries, exports from that country are likely to increase. On the other hand, if goods become relatively more expensive, exports are likely to decrease. This happens because changes in relative prices affect the competitiveness of a country's goods in international markets.