[verzali]

A couple of reasons spring to mind. The obvious one would be this is a person engaged in work that gives access to restricted materials and who also appears to frequently travel to countries like Russia and China, at least from what they have written on the rest of their website. That alone might warrant suspicion of espionage or simple carelessness in handling restricted data. They also seem to be very active in online spaces and possibly they have been skirting close to the line of what they have shared, and thus triggered an alert. It wouldn't be the first time I've been aware of people thinking they are sharing harmless things that in reality are restricted. Of course there are darker possibilities here, and a guilty person presumably wouldn't admit to them in a blog.

Australia is also a member of the Five Eyes. It is not impossible that someone in the US asked Australia to add him to their list.