At the time, there appeared to be a concerted attack on the electronic infrastructure of the economy, which Anonymous was claiming responsibility for. It seems perfectly justified to me that GCHQ was attempting to disrupt those attacks and identify the perpetrators.
Mediatised attack. Their actions are at much smaller scale than any other criminal organization that operates in the shadows (hacking is more profitable than the drug market).
Anonymous is a threat not because of their actions but because they challenge the established powers.
