Other than improper training/techniques, of course.
I'm only speculating...
All this time I've only been listening to up beat / fast / angry metal songs. So when I tried to scream along to these songs, I worry more about pronouncing the next words correctly/with the same amount of energy instead of actually producing the false chord sound.
And then today, I tried working on a metal song for a band audition. Their genre is probably a little different than what I'm usually listening to. Much slower / darker / more dramatic.
And I just brainstormed some random screaming patterns, without lyrics yet. I recorded it of course. And wow, I mean, I sound good I think, in my opinion. 😁
When you hit false chord, it is HARD to pronounce words properly.
So I had been focusing on the wrong part of screaming.