In Will China Deploy Troops in Hong Kong ?, criteria for a deployment of mainland troops are given:

• Defection of an element expected to be loyal.
• Organizing efforts by civil servants, bringing political form to an amorphous movement.
• Emergence of a political movement that attempts to form an alternative government, or simply a parallel power structure.

With some elasticity, the airport takeover could fit the 2nd and 3rd criteria.

The airport is a target of opportunity, for a decapitation strike against an organizational nucleus. A surgical strike also avoids a no-exit conundrum:

• Hong Kong police create a holding line around the airport.
• A mechanized force moves rapidly from Shenzhen, literally crushing those who attempt to impede the move under under APC treads.
• Heavy force, with  some lethal casualties, reminiscent of Tiananmen Square, is applied to immobilize the airport protestors.
• The bulk of the protestors are transported to mainland internment camps, never to return to Hong Kong.
• The mainland force is quickly withdrawn; Hong Kong police deal with the “decapitated” remainder.
• If other strong points emerge, the above is repeated.
• Cleanup and new press laws attempt to erase memory of the intervention.

This is the most likely form. It would be a mistake for the Chinese government to think the vibrant Hong Kong economy would survive. It will not.

While the form of an intervention can be predicted, the likelihood cannot. Whether Chinese intelligence has concluded that their criteria for intervention have been reached cannot be determined from open source.

I feel sorry for the protestors. Those who value freedom so highly should have it.