I agree with Ariana. I think the attack on Bill and her improving relationship with the other Weasleys would've brought the situation too close to home for Fleur to ignore. I can see her being furious that someone had attacked the man she loved; after that, she had a personal reason for getting involved. If the DEs attacked the man she loved, it could be assumed that she wasn't safe either, and so worrying about her own safety wouldn't be a deterrent any more (if it ever was).