Zombie topic resurrection: when someone finds a very old topic that seems to apply to their current problem, and posts a followup asking about the same thing. In one sense, this is good. We like it when people actually search the forums to try to answer their question, instead of asking the same thing that has been asked countless times before. But often, if the thread is old enough, the old answers are misleading and/or no longer applicable. And often what the new post asks isn't really the same question, either because the poster didn't have enough experience, or because of changes that have happened since the original thread.
Hijacking: When someone causes a thread to veer off of the original topic by posting a followup that doesn't really have anything to do with the original topic. The most frequent causes seem to be be a lack of understanding of how to start new topics (for very new users), mistakes in understanding what the topic was about (ie "my failure to upload must be similar to this other users, even though I'm using a different board"), or ... philosophical diversions into interesting but unrelated issues (even very experienced and knowledgeable posters will end up doing this, so that a question about a build error becomes a discussion on C++ revisions, or similar. Sometimes, these are interesting. But probably not to the original poster. (extra points if the OP sticks around, though!)
Now, since you seem to care, let's take a coupe of Arduino-n00b's posts that I found particularly annoying as an example. Second post
The first one was about the loss of the "BYTE" option of Serial.print(). The last post was in 2016, commenting "you must be using a really old version of the IDE." n00b's post complained that some library hadn't been updated; at best, resurrecting an ancient and resolved technical thread with a complaint about a library or perhaps documentation. Maybe not quite a "hijack."
The second thread (about void loop(void) vs void loop() (which concluded that it doesn't matter) is similarly from 2016, and n00b's followup complains about another library and brings up BYTE again, neither of which has anything to do with the original topic. Definitely a "hijack"...
Perhaps what n00b meant to say was more along the lines of "I found a project with these old libraries and they don't compile - where can I find fixed libraries?" That at least might have been answerable.
I'll also point out that neither of the libraries mentioned were Arduino-supported libraries, neither was given a source, and at least the first does seem to have been update (a long time ago.) In the decade that Arduino has been popular, a lot of code has been written, posted, and abandoned by people who went in different directions; that's something you'll have to live with.
So that's why the moderators are annoyed. Ask a question. Follow the forum posting guidelines about what information to provide. Point out mistakes, even complain about bad documentation. Ask for fixed to broken code. But avoid merely whining. Especially avoid whining in very old topics that were about something else...