In "When the Fat Girl Gets Skinny" by Blythe Bairt, what is this phenomenon? Why does it happen? Which ONE line best answers the second question? I need to make at least 20 bullet points answering these questions.

This poem is an unbelievably hard hitting poem. Baird absolutely doesn't hold back, and she gives a visceral and unflinching account of living in a world that seems to pressure people into seeking unhealthy ways to stay skinny or become skinny.
I'm unsure of exactly what the 20 point list should be focused on. If the list is about eating disorders, 20 points could be made mainly by listing a bunch of eating disorders. Baird specifically calls out anorexia; however, there are more eating disorders out there other than anorexia. Bulimia, binge eating disorder, and Pica are just a few. The list could also be a list of all of the fad diets that have been ​in the mainstream in the last 50 or so years. South Beach, Weight Watchers, Atkins, Paleo, and the Cookie diet are all diets that have been established and have people that swear by the diet. There are many others as well. Baird's poem lists out a few of them in the early lines of the poem.

Trying diets we found on the internet:menthol cigarettes, eating in front of a mirror, donating blood.

The list could focus on consequences of eating disorders too. This would include both physical and emotional consequences. What's great about Baird's poem is that she brings up both of them in the poem.

Replacing meals with other practical hobbieslike making flower crowns, or fainting.Wondering why I hadn't had my period in months
[...]
because I only feel pretty when I'm hungry.If you are not recovering, you are dying.

It's also very interesting how Baird lists some "positive" consequences too. Just reading the poem might not drive home how snarky and sarcastic some of her lines are, so watch her deliver the poem, and you can really feel how cutting some of her remarks are about her friends and family. A good example of the "positive" consequences of her eating disorder is how her dad reacted. He's so proud of her thinness that he carries before and after pictures of her. He's also relieved that obesity related diabetes isn't something they have to worry about anymore. He's proud that she is "taking care" of herself. The dad has no clue that her eating disorder is slowly killing her.

When I lost weight, my dad was so proud, he started carrying my before-and-after photo in his wallet.So relieved he could stop worrying about me getting diabetes.He saw a program on the news about the epidemic with obesity, said he's just so glad to finally see me taking care of myself.

Baird's poem discusses some possible reasons as to why eating disorders are used, and the 20 point list could focus on that angle too. There is no definitive answer as to why eating disorders are used, but Baird very clearly points out that often an eating disorder is triggered because of body image concerns. Baird has no problem calling out the media industry, either, as a main influence over a young girl’s body image confidence.

Watching Americas Next Top Model like the gospel.

Baird's poem points out that media images deserve part of the blame for the problem; however, her poem spends more time pointing out and criticizing the influences of peer groups. She specifically points out friends congratulating each other on "thigh gaps." She also points out that her popularity increased because she dropped weight:

So when I evaporated, of course everyone congratulated me on getting healthy.Girls at school who never spoke to me before, stopped me in the hallway to ask how I did it.I say "I am sick".They say "No, you're an inspiration!"

Baird's family's reaction is a strong encourager of the disorder too. Baird saw herself as skinny and more like the models on TV all while people are complimenting her thinness. She fell in love with her new body image and the compliments it garnered, or she was afraid of returning to a life of being ignored for not being thin. Either way, she linked the disorder to being popular and interesting.

Why would I ever want to stop being hungry, when anorexia was the most interesting thing about me?

As for which one line best highlights why eating disorders exist and are pursued by people, that will depend on individual readers. I definitely think the America's Next Top Model line is an important line. The line uses the word "gospel," so it carries biblical weight. For Christians, the bible is the word of God. The gospel news points toward the work of Christ and the salvation he offers. For Baird to equate a show about skinny women to gospel truth is an important message. I also think that any of the lines that point toward peer pressure do a good job of explaining why a person would resort to an eating disorder. But if I have to choose one line, I choose the following line:

As a child fat was the first word people used to describe me,which didn't offend me, until I found out it was supposed to.

The peer pressure wouldn't exist if nobody saw fatness as something to be insulted. The media influences, family pressures, and peer pressure are results of something that has been taught to us since childhood. Fatness is bad. Skinny is good, and the end result justifies the means.
https://genius.com/Blythe-baird-when-the-fat-girl-gets-skinny-annotated