A doorway page is an old school black hat SEO technique. What webmasters would do is buy domains with high PR or buy expired domains that used to be competitors and then 301 redirect them back to their website. This was in essence buying their links, as the links to the old domains now ended up at their domain.

Are your domains all on the same hosting account or same serer c-block? Are they all registered and verified with Google Webmaster Tools? If not, then Google may seem them as being owned by different people. In that case, it would look to them like you just bought a bunch of domains and redirected them all to your domain.