They weren't just taking back their property after a lease ended. The UK had leased the New Territories, but the Kowloon peninsula and Hong Kong island had been ceded. When the lease expired, the UK could have given back the New Territories and kept the rest, but it was agreed to hand over the whole lot.

That agreement was part of a deal which also included "one country, two systems".

China made these promises in part to assure the residents of Hong Kong (and international observers) that their rights would be respected, and also to prevent economic collapse caused by businesses deeming the island an unsafe place to do business.

What most people call Hong Kong consists of three major areas: Hong Kong Island, Kowloon, and New Territories.

Hong Kong Island (where majority of people live) was given to UK with no lease limits. Kowloon and New Territories were leased to UK for 99 years in 1898. So, in theory UK only needed to return Kowloon and New Territories. Of course, this wasn’t practical. It’s like giving Manhattan to another country but keeping Queens and Brooklyn. So in 1994 UK agree to return all 3 (Hong Kong Island, Kowloon, and New Territories) in 1997.

Well they did sign an internationally binding agreement over how Hong Kong would be treated after handover.

They already made the promises, and lodged them with the UN.

Now they are breaking those treaties.

To ensure a smooth transfer and a certain degree of self-determination for the people who lived there, were reasonsably content with the existing set-up and would have been most unhappy if the UK had simply washed its hands and disapeared.

Because they signed the Sino-British Joint Declaration: https://en.wikipedia.org/wiki/Sino-British_Joint_Declaration