My take regarding the underlying grievances of the protests, that started years earlier:

-socio-economics already explained in other posts here, -fear of an increasing authoritarian China and encroachment, (btw anyone heard of concrete influences/violation of Basic Law into HK before and during protests, aside from the bookseller abductions?) -psychology, identity (changing relationship between HKers and Tier 1 mainlanders in terms of being special, and competition for jobs) and -local policy/policing mistakes that were re-enforced by populists and social media bubbles eg -local economy and services being transformed by businesses catering for an increasing number of visiting mainland tourists and the little benefit that gave to the ‘normal’ HKer in contrast to tycoons), -the stress to social services perceived due to immigration of mainlanders (eg gaining HK ID cards with crossing the border in late pregnancy), -the issues illegal parallel/cross border traders brought - eg during the milk powder shortage. -the mainland ‘dama’ singers...

-sensationalized individual cases of mainlander ‘poor behavior’ - eg that defecation of a child in the streets, or some eating behaviors ....feeding a narrative of separation, tribalism.

-slow progress towards more political participation/universal suffrage - perceived to be a right under Basic Law - but debatable IMHO.

All this contributed in various ways until it bursted out and kept escalating also driven by the poor government response.