You did not mention the reasons given. However, common reasons could be:
1. If text in your hub resembles to text on the web, there are possibilities that matter has been copied from the web or someone copied your hub and published somewhere else. It will show content copied even if this content has been published by you.
2. If objectionable keywords are found in the content, it is considered objectionable to advertisers. Hence, objectionable keywords detected by automatic filters or manually moderation makes it possible for pulling.
3. Brief text (less than 1oo words) also disqualifies any hub for publication.
4. More than two links pointing to same site (barring educational & few other categories of sites) is considered as promotional and a hub may be pulled.
There are several other conditions that can be read from HP help.