I can't speak to the phenomenon as a whole, nor can I even explain the causes of my own fat fetish, but my experience suggests that while it is partially rooted in culture, it is also something which is genetically encoded.
When I was five, I was watching a cartoon where a female character mentioned that she was getting fat and even without an accompanying visualization, I was immediately aroused. After that, any time a woman or tv or one of my mom's friends would say something similar, I would just be fired up. Several years later, when I was 10 or 11, a family friend came over and had noticeably gained weight. Six months before, she had been long and lean to the point of being stringy. Now, she had a full-blown potbelly. She caught me staring and though I now realize that she probably enjoyed the recognition, at the time I was afraid that she might have mistaken my look for one of shock or disgust. After I had looked, and I imagine that my parents had given her some looks as well (they were shocked by it later when I overheard them talking), she said that she had simply let herself go and that she was just enjoying her life.
It was only after that incident that I understood that weight gain could be arousing to someone other than me and was therefore in the realm of the socially possible. But even without that event, I'm sure that I would have learned to articulate my preference sooner or later, perhaps in a relationship where some unintentional gain had occurred.
I do admit that I find the "naughty" aspect of weight gain and being fat to be appealing, but that is centered in the physical pleasure that seeing round bellies, big butts, and thick thighs gives me.
