What Exactly Triggers A Google Penalty?

Being penalized in Google is something that happens, and not infrequently.

Consider these suggestions from their guidelines:

• Avoid hidden text or hidden links.
• Don't employ cloaking or sneaky redirects.
• Don't send automated queries to Google.
• Don't load pages with irrelevant words.
• Don't create multiple pages, sub domains, or domains with substantially duplicate content.
• Avoid "doorway" pages created just for search engines, or other "cookie cutter" approaches such as affiliate programs with little or no original content.

This is basically a list of "don'ts" whose existence proves their success. You'll get penalized in Google if caught doing any of these things.

There are other ways to achieve a Google penalty, like buying and inter-linking too many domains, listing your keywords repeatedly, linking to a bad neighborhood, accidentally exposing (through links) parts of your site that should not be live, etc.

This list came about by tactics that have been used in the past  (and some are still used) to enhance page rank. These tactics usually can get high rankings quickly and lose them just as quickly once found out by Google.