It's just what HKG does with US bound flights. Extra screening at gate lounge for all pax, dumping any liquids.

I was flying HKG-JFK. I had picked up a bottle of water from The Wing before I headed downstairs to the gate. The HKG contract security told me I had to toss the bottle. I drank it instead.

Most countries don't ban liquids. Flights to the US aren't allowed to have liquids. If flight isn't screened to US requirements, it isn't allowed into US airspace. Thus many airports screen US bound flights at the gate for the extra US demands.