TL;DR - Anderson didn't want to go southpaw, Amanda threw a feint right before the combo that totally fucked Anderson up and made her go southpaw.

Full analysis:

Ok here's my take on what happened. If you slow-mo the gif, Amanda throws a quick feint right before the combination that Anderson thinks is a leg kick. Anderson checks the feinted kick by lifting her front leg. The foot then lands out to the side so she's square to Amanda instead of landing back down in front. This was the mistake that fucked her up so bad. To fix that mistake she takes a panic step back, trying to switch her stance to southpaw.

Here's where shit goes terribly for Anderson: When she realizes she bit on the feint, (and while trying to switch to southpaw to fix her mistake) she tries to pre-emptively counter the punches she knows are about to blast her. You mentioned in your first comment the "lead uppercut," but immediately before that she threw a really bad left (7.2s). If you look at that punch and her upper body rather than her stance, it's like she tried to throw a left hook uppercut combo, but she's so off balance it's absolutely terrible. You can tell by the way she pivots her left leg in the air as it's moving backwards (7.2s) that she's all fucked up and technique is out the window.

That's why she's in such a bad position and why the punch looked absolutely terrible and she had nowhere to go. She bit so hard on Amanda's feint that her upper body is trying to throw an orthodox combination (lead left -> right uppercut) while her lower body is trying to switch to southpaw. She's trying to rotate her left shoulder forward and her left leg back at the same time, ending up with one leg twisting wildly in the air attempting to throw an off-balance counter combo while she's getting blasted by Amanda's bread and butter jab overhand combo.