As a fatfag myself, I'll explain.
I'm most likely mentally ill, but my fat fetish resonates from when I was a little boy, watching cartoons that featured episodes depicting weight gain/feederism so that's one cause. Another might be due to my mother, who was a overweight woman herself, so maybe my kid brain registered this as "oh, this is how a woman should look" or something like that.
The last cause is a moment when I was 12 and my cousin who was overweight, sat on top of me and refused to get off until I mustered up the strength to push.