Violation of Google Quality Guidelines is a major issue these days. Google is rapidly deindexing website found guilty of violation. Deindexing of website is just to make search results more relevant, rich and high quality for visitors.
In my opinion, following are some major reasons for guidelines violation:
(a) We are unaware of guidelines.
(b) We are aware of guidelines but don't understand them in technical terms.
(c) People are participating in link schemes, reciprocal and paid links just to rapidly increase PR and traffic.
(d) Making website for search engines not for visitors.
(e) Use of scraped, syndicated and copied content just to increase search results.