It's a whole combo of factors.
Mariah should have been there for the rehearsal instead of a "body double"
The audio engineers should have made sure her in ears were working. They should have played Emotions with a lead vocal. BOTH parties should have agreed on the songs being performed, clearly there was an issue since Mariah commented they hadn't gone through Emotions beforehand.
At the end of the day this was inevitable for her, we all saw how her shows and general showmanship was making a turn for the worse.