Disturbance at the Heron House is written in Michael Stipe's typically oblique fashion, allowing many interpretations and enforcing none. Document was R.E.M.'s most overtly political album, following closely on the heels of Lifes Rich Pageant (no, there's no apostrophe in that title), which was the bands first album with any overt political content.
The songs on Document tend to focus on labor struggle and political repression, specifically Finest Worksong, Welcome to the Occupation, Exhuming McCarthy, Its The End Of The World As We Know It, (And I Feel Fine) (again, no apostrophe in that title), Fireplace (which I've always felt is about revolution), and Disturbance at the Heron House.
A common interpretation of this song is that it describes a political protest and/or labor rally from the point of view of the elite.
"The followers of chaos" being one of the elite's derogatory assesments of the protesters, along with "the green and simple", "grunts and greens", and "cogs and grunts and hirelings". The "mean idea" being the protesters supposedly simplistic and erroneous beliefs.
A metaphor of a zoo ("they've gathered up the cages") is employed to illustrate the elite's view of their inferiors as simple, mindless animals ("a stampede at the monument"), and expect that they will ultimately submit when the sacrifices made for the protest become too great; "when feeding time has come and gone, they'll lose the heart and head for home".
The elite are unmoved, unsurprised and unimpressed by the lower classes' latest attempt at fundamental change; "try to tell us something we don't know".