Why did Qianlong abdicate in favor of his son Yongyan?

Qianlong abdicated in 1796 primarily out of respect for his grandfather, the Kangxi emperor, whose reign had lasted 61 years. Having reigned for 60 years himself, Qianlong chose to step down to ensure his own rule did not exceed that of his predecessor.1

Although he designated his 15th son, Yongyan, as his successor, Qianlong continued to hold real power within the palace until his death. Consequently, the customs of the Qianlong era remained in place, and the new emperor, who took the reign title Jiaqing, spent most of his life in his father's shadow.12