First question, what is Trinity Syndromehttps://thedissolve.com/features/exposition/618-were-losing-all-our-strong-female-characters-to-tr/
If you read the article linked, you now know what Trinity Syndrome is.
I have been thinking about this for a while, it's been simmering in the back of my mind.
There are two things that have stood out, Supporting Character Role, and Half-assed attempts.
The latter is the larger problem, it has been rather traditional for the characters in stories to be white men, but up until recently, most of Hollywood was populated with just white men, and they made the roles, and everything else. The Half-Assed Attempt here is that the story tellers and writers and producers are making an effort to make better female characters, they are making Valkas, Dahls, Trinitys, and so forth, but they are still cleaving to the Hollywood standard of making the main character white/male. There is no reason that prevents Hiccup in How to Train Your Dragon from being a girl, no reason for the One of the The Matrix to have been Trinity, with Neo being the supporting character. It's the half assed effort, and it's changing, but you can see the drag.
More evidence supporting this observation:http://www.cracked.com/article_19549_5-old-timey-prejudices-that-still-show-up-in-every-movie.html
The former is an unfortunate aspect of writing and storytelling. It is up to the main character to overcome the foe, solve the problem, and save the day, make the internal realization, and otherwise be the focus of the story, they are the main character, after all. The supporting characters come in to play their roles relative to the main character and the story. The mentor, the specialist, the badass, the token character, etc, they are all SUPPORTING. If Obi-Wan told Luke to go home, and went on to Mos Eisley without him, there's no story because Obi-Wan is not the main character. This is normally not an issue, but it becomes one through the lens of social justice. With the half-assed effort, we are seeing more strong female characters, more strong ethnic characters, but they are in supporting roles, and as such they have to be unable to resolve the conflict of the story.
Thus, when the strong supporting female character fails, the shouts are raised. When the ethnic supporting character is killed by the baddie, shouts are raised. If either character had been a hollywood standard white guy, no one would care.
How do we address this?
Admit the role of the supporting character is not to overshadow the main character. They are mentors, rivals, allies, friends, etc, not dues ex machine to win the day.
And write better. Don't punk supporting characters, don't cut them off 'just because', and when they falter, fail, or perish, while it is functionally to serve the story, it must never appear that way. If Princess Thundera has been beating up baddies alone for a while, and the MC shows up, she shouldn't fail just because the MC has decided to show up. There is too much of that in the movies as it is. Another prospect is to not be afraid to make leading characters not functionally white men.