The first question that pops to mind is whether that material was published elsewhere on the web. Have you published it anywhere else such as another writing forum? If you have, hubpages will mark it as a duplicate whether you are the original author or not.
Have you used a lot of quotes or exerpts? That could be the reason also. Hubpages would this as duplicate material.
There is also the possibility that someone has taken your material and published it elsewhere. If this is the case, hubpages will help you find the person who took your work.