It's likely not performance concerns. I suppose she just monitors her logs meticulously and notices stuff. 404 errors can be a witness of a broken link she should fix on her site or get fixed on external sites. She probably looks for them and notices URLs that look credible. She might still look for broken RSS readers too. Or just suspicious things.

I highly doubt that the extra traffic is really the problem, it's merely the catalyst which caused this post to be written.