I'm assuming you've never taken any drugs that induce what they call "ego-death"? Basically, certain drugs send you on a trip inside your own mind and teach you about who you really are, inside and out. It can be terrifying if you suddenly realise that you're selfish, greedy, angry, cruel etc... You get to look at yourself from an outside perspective, without your ego telling you that you're perfect. For this woman, she suddenly looked at her body the way others see it, as a disgusting blob of fat! As /u/Patarack stated, imaging going to sleep as you are now and then waking up in an enormous, fat body. Then imagine that you've been living in that body for a long time thinking it was perfect. That sudden realisation would be horrifying and that's what happened in this video.