Sysop vandalism is a common phenomena on large public wikis. People who are unwisely granted sysop status and priveleges abuse them to do what, if anyone without such powers did it, would be considered wiki vandalism.
Historically there is a long history of official vandalism in which people who have been granted official posts use it to damage works by others who are officially disapproved of. Good examples include the Chinese Cultural Revolution period, and the defacement of Jewish synagogues and cemeteries by the Nazis just prior to World War II.
The much more trivial wiki version of this phenomenon is typically carried out by a sole person unqualified to be a moderator who received the powers to carry out some technical function. Once granted this power, they may begin to push an agenda or carry out a campaign of censorship using it.
It is closely related to, but a different phenomenon than, sysop vigilantiism. Often the two occur together, e.g. vigilantiism to block IPs followed by destruction of the useful material they contributed, even though most other users consider it valuable.
An amusing example of this phenomenon is the article (or non-article) on this topic at Wikipedia. The phenomenon very clearly satisfies the criteria for inclusion in Wikipedia, being in widespread use to describe a common phenomenon, but the actual power abusers (many of whom favour right-wing views of Zionism and George W. Bush) repeatedly delete it.