Just because the promos showed a clip of Tiffany crying after the yelling doesn't necessarily mean it happened in that order- they cut and paste all the time.
I don't think it would be someone like Rebecca, just because she doesn't improve each week. That's a different kind of "yelling". Based on what Tyra is saying (as she yells), it would have to be something major (or more than usual) to provoke her. I can't remember exactly the words she uses but I kind of remember her saying "be quiet!" at the beginning of the promo they show on UPN.
I want to say it's Tatiana, Brittany and Tiffany because they are the ones that would be "talking back", in order for Tyra to say "be quiet" (if she did actually say that). They are the more combative girls (now that Brandy's gone).
The ones that I wouldn't bet on to be the girl is Rebecca, Kahlen, Naima or Michelle. Christina... well, last week when she broke down and cried- I felt a little like she was giving an excuse (i.e. talking back but doing it in a different way)- but I don't really think it's her either.
That said, I wouldn't think it's Tatiana, only because they focus on her expression after the clip of Tyra yelling. Too obvious.