According to a study "Delinking: An Exploratory Study" by Isola Ajiferuke published in Webology, Volume 5, Number 1, March, 2008 most web masters have to remove external links from their sites. The most common reason for removing links is obviously that the linked pages cease to exist or had become irrelevant. However sometimes the links are removed on the basis of Political/philosophical difference with owner of linked site.
..webmasters, especially those at colleges and universities in Canada and US, do delink on a regular basis, and reasons for delinking include dead links, site outdated, site is poorly designed, and site’s content is of low quality. Given that most web search engines use the number of inlinks as one of the criteria in ranking search output, a site owner/webmaster should try to minimize the risk of the site being delinked from by other sites by ensuring that the site is well designed, rich in content, up-to-date, and if the site’s (or any of its pages) address changes, users are re-routed from the old address to the new address..