The demand for HK dollar reflects income, interest rates, inflation, business activity and sentiment in Hong Kong. It also appears that a significant proportion of HK dollars are held outside HK.
Hawkins, J., & Leung, C. (1997). The demand for Hong Kong dollar. Hong Kong Monetary Authority Quarterly Bulletin, May 1997 (11), 2-18. https://www.hkma.gov.hk/media/eng/publication-and-research/quarterly-bulletin/qb9705/fa01.pdf