Those say sysops are trolls consider this a form of troll vandalism. It is at least group vandalism. From Sysop Vandal point of view, however, there can be no such thing as sysop vandalism since the two categories are mutually exclusive. See also New Troll point of view.
Sysop vandalism is degrading the quality of a blog, newsgroup or wiki for reasons that have nothing to do with the objective of the project or the mandate of the group. It occurs frequently on Wikipedia and Disinfopedia - it is worst on the latter where sysops tend to delete things with no process at all, and ignore the votes for deletion process that non-sysops are required to go through. This is a very severe long-term problem and plays a major role in many worst cases visible on other public wikis.
Such vandalism is surprisingly common: Wikipedia permits and encourages sysop vandalism with the universal excuse that "trolls" were responsible for the edits, and that somehow they will be discouraged by being "punished" somehow - this idea that punishment works is part of a carceral state metaphor. This is the basic conceptual metaphor of Wikipedia and other wikis that insist one use real names (and thus be subject to offline abuse by sysops and their friends). There is no definition of the hated class of trolls other than by the very sysops who exclude them, and the developers who provide features to do so. This is the first step towards a permission-based model and there may be no way to avoid imposing hard security if this path is taken by developers. Mediawiki does seem to be evolving in this direction.
These problems are made much worse by permanent sysop status and a model where one pays no price and loses no status even for the reversion of edits which are deemed ultimately constructive. Such behavior is certain to drive off the best contributors, but to serve the sysops' purpose of "converting" the board or wiki into a virtual community only for their own friends.
The most common sysop vandalism is ad hominem delete or ad hominem revert of legitimate facts or analysis, or fair comment, simply "because that user is a troll", whatever that means. This is enabled by the infamous "rule 6" of the candidates for speedy deletion page, which remains despite being hotly contested by all who understand systemic bias. This policy is for the convenience of the sysops, and not the users. Users of course expect all information that advances the mission of the wiki to be available, at the very least in the older versions, and the GFDL seems in spirit at least to require this, in its attribution terms, in its requirement that modifications be share-alike, and its assumption that some moral rights apply (like retrieving the source text of your own article, or indeed any article with "no technical barriers", i.e. no IP block, no MySQL).
Typically such vandalism is a symptom of conflicts between users in which the sysop is not neutral, that is, they wish to encourage one contributor and discourage another. They abuse their sysop powers by banning "those who their friends do not like", and eliminating valid contributions towards the goals. There may be cases where this is valid, i.e. someone irreplaceable has made clear that they will not participate if someone else is tolerated. Unless the medium has a formal power structure, e.g. it's a political party large public wiki governed by the policies of that party and its officers, such decisions are almost always either "sysop instinct" or GodKing choices.
A specific sysop power structure may be required to reduce such vandalism.
A related issue is sysop vigilantiism which is not necessarily vandalism, but does subvert due process and degrade trust in the power structure. However, social exclusion of some faction can just cause it to go somewhere else: as with Wikipedia factions, many of whom left early to do something else. Sysop vandalism is much worse, because it prevents the actual GFDL corpus from being corrected, and may not even be noticed for a long time. So controls on sysop vandalism are highest priority, as with all other vandalism (the only reason to need any kind of sysop).