I had this exact issue one time too - to figure out what it is, click on the individual hub and go in to 'edit' - at the top it should tell you exactly which link is the problem. In my case, it turned out that I was linking to a site that was considered too 'promotional' content-wise, even though I did not actually have 3 links to any one site. I guess it considered that site too promotional in and of itself (even though I only linked to it once), so I simply deleted that link from my hub. It was a bit annoying because it was one of two sources of research that I had cited, but I didn't really have a choice. Remember that even when you have fixed the problem it can take a few hours for the red skull to go away. Keep us updated.