Why Were They Arrested?

Some 1.7 million people showed up for a rally in Victoria Park on August 18

Can someone explain this to me?

Yesterday the Hong Kong police, on orders by the government, arrested 15 high-profile people for allegedly organizing and taking part in illegal protests, including one on August 18 last year against the extradition bill.

That day the police had only given permission for a rally, not a march, but how do you squeeze 1.7 million people into Victoria Park in Causeway Bay? They had no choice but to walk out the area to allow more protesters into the rally.

That evening, after some 1.7 million people protested, the government released a press release admitting the "public meeting was generally peaceful", though participants occupied a number of thoroughfares on Hong Kong Island, causing inconvenience to the public.

There is no mention in the press release that the gathering was illegal, that whoever organized the rally would be held to account.

So why were the likes of Martin Lee Chu-ming, Margaret Ng Ngoi-yee, Lee Cheuk-yan, Leung Kwok-hung, Jimmy Lai Chee-ying, and Albert Ho Chun-yan arrested again?