It's a bit of a signal in some religious/evangelical language. "new atheism" implies a few things - there's a renewed urgency, an implied "new" "attack" on religion, etc. It's not meant as an insult so much as a buzzword to signal to the faithful that there's a 'new' menace to ward against.

Also, 'new' atheism is viewed as actively oppositional - loud, proud, 'coming out' in public, etc. Being aggressively atheistic is a hallmark of those labelled 'new atheists'. It's probably more accurate to refer to this more aggressive public movement as antitheism, but that would be another label/word which would confuse people. Putting 'new' in front of the old word, I think it becomes easier to rally the troops, so to speak.