What are the possible mechanisms that could cause a rapid drop of temperature over few days?
If you look at the first picture, it is the barometric (atmospheric pressure) map of the Honk Kong region for 12 October 2015, from the Hong Kong observatory website. Barometric maps don't change radically over the a period of a few days. The second picture is a similar map for the south east Asia region as at 8 am on 11 October 2015. It is from the same web page as the first picture, I just selected an earlier date from the upper right of the page.

Some things to note about the first picture:


Temperatures in Beijing, Seoul and Hong Kong are 12, 10 and 21
degrees Celsius respectively. It's much colder in Beijing and Seoul than it is in Hong Kong.
There is a high pressure system to the north of Hong Kong and to the
west of Shanghai which appears is part of a larger high pressure system WNW of Beijing. There also appears to be a low pressure system to the
north of Japan (which is confirmed in the second picture).


In the northern hemisphere, air flows clockwise around high pressure systems and counter-clockwise around low pressure systems.

The affect of the high pressure systems over China combined with the low pressure system to the north of Japan is to funnel cold air from the northern latitudes to the southern latitudes. This brought a significant amount of cold air to Hong Kong which caused the temperature to drop.

If you look at Manila, in the Philippines, it is a relatively short distance from Hong Kong. It's temperature is 27 degrees Celsius compared to 21 for Hong Kong.

If you look at the first picture, there is a low pressure system just to the west of the Philippines. With air moving counter-clockwise around low pressure systems, warm air from the equatorial regions is being drawn northwards along the western parts of the Philippines maintaining a very warm temperature in Manila.

Currently, Hong Kong and Manila are being influenced by different weather system and that's why there is a significant difference in the temperatures between the two locations despite their close proximity. 



Barometric Map of Hong Kong for 12 October 2015


Barometric Map of South East Asia as at 0800 11 October 2015