I think one of the most common (and global game effecting) toxic factors on mushes is when a large group with power/influence starts to defend things as being the terf/right etc of people within such group or on good terms with such group and starts to /other/ people who are not liked by the group or who are outside the group. The othering can come with being blacklisted/stonewalled/bullied and etc.
This can include thinking that certain ic positions, titles, plots and etc belong to certain players and that other players should not have access to them and if they do have access or pursue such they are somehow taking from the players who deserve such more.
Once someone talked on a public channel about wanting their character to be a demon hunter and was immediately jumped on by people going no that this the domain of so and so and it would be ooc wrong of you to ic pursue to that because that plot is the property of so and so. The person jumping on this person had great influence on the game and the person they said demon hunting should be reserved for was their good ooc friend was in with their powerful group. The person who was saying they would like to pursue being a demon hunter was outside the group and not liked by the powerful people on the game. I have seen this time and time again. If someone in their group had said they wanted pursue that the response would probably would have been completely different.
That plot belongs to so and so.
There are limited people I can take, so I will take five characters played by my ooc friends.
They don't deserve such because they are too new, didn't work as hard and etc.
If you take my character on this, I will have my alt bring your character on their next thing.
So and so might feel hurt and quit the game if they don't get such and such. You pursuing such and such plot line is wrong and hurting that person ooc.
Everyone thinks this plot should belong to such and such.
Everyone will dislike you if you run for such and such title.
Oh your character has violet eyes and white hair like mine? Clearly you are copying so and so in our group and its not right.
We don't need a healer xyz is already the healer and they been around longer than you. In fact you are ass for even making a healer in the first place.
Etc, etc.
Generally the people reserving things for their group sincerely believe they are doing a good thing. They see how hard their friends work for things. They feel like /others/ are stepping on toes. They look at rp and plot as limited rather than expansive. As someone else getting involved will take away a slot from one of their friends, not as in roleplay and increased involvement of others creating more rp and story for the game at large.
One person being insecure and defensive about their plot can be annoying, but generally doesn't wreck a game. However I noticed that some mushers can be even more intense in their plot-hording, bullying and etc when they feel something is being taken from the friend than from themselves. Sometimes people can get very intense and passionate when defending friends. A group of people who are defensive about things belonging in group can be seriously damaging to a game, especially if they are playing multiple high powered characters with positions across spheres/factions/house etc. It can be this way even if staff are decent.
I am not sure what the answer is, but I think limiting people to one alt with a big title could help reduce one group dominating the powerful positions. I also think that rewards for bringing different players who are outside one's group into plots/scents etc could be helpful. I also that allowing a character to be close/allies with only one alt per player. So it is not like all the alts of players in a certain ooc group are allies with all the alts of the those same players and etc. And I think that people should be limited to have one alt hate/dislike/go against a certain character. To help prevent the this group doesn't like me ooc so all their alts are jerks to me effect.
But then I think that awareness helps too. For people to be aware of this and not surrender to a powerful group, but at same time to keep their cool and calm.