There are various reasons for getting banned in Google. If you are using blackhat techniques like, keyword stuffing (dumping kws with same font colours as background), doorway pages, cloaking, redirecting sites to other sites etc. Duplicate content, having links from bad sites or spamming sites and also from getting links from already banned sites.
Anything that you do to confuse the search engines and their algorithms is an open invitation for getting a site ban from them. It is mainly related to the practices you use to get higher rankings in SERP mostly using unethical ones.
sad thing is that some sites do cloak and have doorway pages, but are so big that google just steps aside and makes acceptions. Biggest site that comes to mind is run by the new york times...
Here's a funny blog post that highly relevant to this topic http://blog.outer-court.com/archive/2006-12-11-n75.html
You definitely have to work with Google and not work for Google or try to make Google work for you. That's vague enough to put in the new edition of Google guidelines.
May be due to the copy of content from others sites or get back links from banned sites which is already ban by google.
Black hat techniques, if detected, you may recieve a penalty or be banned until you ask for reinclusion.