It's very likely that Tianna simply became overwhelmed with the tour. I've never been part of a touring company, but I imagine that it gets very stressful. I most certainly would NEVER be able to pull off what Tianna and all the Annies of the past have managed to pull off. I remember Andrea McArdle saying that she pulled a prank on Reid Sheldon, telling him that she couldn't do the show that night, that it was just "too much", and that he believed the whole thing. Although she had been totally pulling his leg, the idea of a small child being overwhelmed by audiences of 2,000 people is not unrealistic.
However, the person who originally spilled the beans yesterday (the post that was removed) said that Tianna's mother received a letter saying that Tianna was fired. That person could very likely have been embellishing the story, but considering what happened in 1997, I wouldn't put it past the producers to fire a child.