She was not arrested, but "detained for security reasons" which means she was not violating laws, but just removed from the place by "force" and registered by the police so they could demolish the village Lutzerath so the browncoal mine could expand. Less than an hour after this happened she was free again and protesting a bit further (behind the fence surrounding the mine) from the open browncoal mine where this happened. And as she is the face of the movement, camera's focussed on her. But there were a few tenthousands of protesters arround that mine. Here is a picture of the place of protests where she was removed.
And most of the thousands of protesters there in Germany were removed like that, only a few dozen were arrested, mostly because of violence against the german police. That happened when the german police raided the occupied village itself.
And now back on topic