It was a day of hemorrhaging led by property stocks.
The stampede came after Beijing reportedly told major real estate developers in Hong Kong that the game rules had changed so much so that the age of monopoly had ended for them.
The report said they were now expected to commit resources and influence to supporting Beijing's interests, while at the same time helping overcome the SAR's housing shortage.
It was a blunt message revealed by Reuters.
Despite the lack of official confirmation, the report appears credible in the sense that the news agency said it was based on interviews with three major property developers and a government adviser.
The SAR faces what could be a game-changing moment, so it's small wonder investors sold first, preferring to ask questions later.
On the first trading day after Reuters broke the news, Henderson Land plunged 13 percent. Sun Hung Kai Properties performed a little better, still diving 10 percent, while CK Asset sank 9.32 percent.
All in all, the Hang Seng Property Index fell 6.7 percent.
During the day, it was common to hear analysts blaming the intensifying China Evergrande debt crisis for the sell-offs.
Though they might not be wrong in pointing the finger, the Evergrande crisis has been known for some time.
What stoked the Black Monday sell-offs were fears that Beijing may extend its "common prosperity" agenda beyond the border to the south and apply it to the SAR too.
The Reuters report did not say exactly when Beijing lectured the Hong Kong property tycoons.
However, they recently acted in concert - either donating land or establishing a partnership with the government to provide public housing.
For example, New World and Henderson Land donated from their rural land reserves a handful of plots to be used for social housing. Others, including Nan Fung Group and Wheelock, applied for the public-private partnership scheme that was unpopular to the sector.
Obviously, Beijing wants more than these goodwill gestures.
It would be unrealistic to bet that the new policy will again be short-lived as previous ones usually were.
Since the start of the year, the administration has been exercising its power to resume land for public purposes.
Overall, a total of 637 plots were reported to have been resumed between January and July, allowing the government to possess more than 26 hectares of land from private ownership in Tung Chung, Yuen Long, Fan Ling and Tseung Kwan O.
Major developers in the SAR have stocked up millions of square feet of farm land in the New Territories.
For instance, Henderson Land has a reserve of over 40 million square feet of farm land, followed by Sun Hung Kai Properties with about 30 million square feet.
If the administration had been careful not to readily wield its power, it has now shown a readiness to exercise this authority to increase land supply for public housing.
As said, the rules of the game have changed.
Will the paradigm shift be spelled out in clearer terms in the October policy address?