Only some websites are placed in the sandbox, but it happens more frequently for those websites which are seeking rankings for highly competitive keyword phrases. In order for your site to get out of the sandbox, you need to continue building strong incoming links, have good link anchor text, and add more keyword rich relevant content on your webpages.
Google does not say much about sandbox effect, and when you search for sandbox term you will find Google Adwords Keyword Tool as the first result.