Smallpox and civil war are two things caused a decline in the Incan population prior to Pizarro's return from Europe
Step-by-step explanation:
The Incas were at that time operated by an epidemic of European diseases, such as smallpox, which had expanded from central America still more active than the European intruders themselves. Such a disease hit Wayna Qhapaq in 1528 CE and any spots a monstrous 65-90% of the residents would fall from this invisible enemy.
In 1533 CE the Inca Empire was the greatest in the world. But, the absence of the combination of succeeded peoples into that empire, combined with a civil war to claim the Inca throne and a devastating epidemic of European-brought diseases, meant that the Incas were ripe for the taking.