Google often sandboxes sites that provide small or no value to the users:
Very old sites, that no one maintains get sandboxed.
Spammy sites, centered around one keyword repeated thousands of times too.
Sites that are a security risk or start auto-download viruses on entering get sandboxed and do not show in search until fixed.
(Additionaly, such sites get a small intro window with "I know the risks" button, that don't allow you into the site)
Sites with copied content get sandboxed very quick
(Google keeps archive of all sites and knows very well WHEN and WHERE an article came to life first. All others are copies and a subject of sandboxing)