Why were water bottles confiscated in a boarding gate jetway in Hong Kong?

Seeing another question about taking water bottles onto planes reminded me of an interesting incident I ran into once. During a layover in Hong Kong in late May of 2013, as we were boarding a flight to Los Angeles, there were security personnel actually in the jetway searching through everyone's carry-on luggage and confiscating water bottles that still had water in them. Since only empty bottles were allowed to get through security or transfer security to even get to the gate, this means that all of the water being confiscated was either purchased or obtained from a water fountain in the airside concourse. As such, I was confused as to why it was being confiscated. Also, this was not done on a different departure I had from Hong Kong only a couple of weeks before that or on any other flight I've ever been on anywhere.

So, my question is, does anyone know why they would confiscate water that was obtained in the secure departures area of the airport in the jetway as passengers were boarding a flight?

The only even remotely reasonable explanation I could think of is that it could have something to do with taking extra precaution to prevent the spread of avian flu, since there was an outbreak in China at the time, though I didn't hear any kind of announcement of this not being allowed until we were already actually in the jetway past the boarding gate.

Less likely, this was during the time that Edward Snowden was in Hong Kong, so perhaps this was a pretense to search luggage of U.S.-bound flights for some reason related to that, but this seems very unlikely, especially since this was between the time he arrived in Hong Kong and when he went public with the information.