I started getting them in my teens with no family history. The typical medical response is that they are “benign” but I personally don’t believe that. They may be benign in the limited sense of “not cancerous” but they represent a dysfunction in the body; whether that dysfunction is completely harmless is doubtful.
As for what causes them my own experience is that they multiply whenever I have extended periods of intense physical activity (extended being many months). So I am partial to the theory that they represent a dysfunction in the body’s healing process, likely in the mechanisms through which creatine mediates the disposition of fatty tissue (a process known as “futile creatine cycling”). However, that is just a theory. As you point out it is an uncommon disorder, widely viewed as harmless, and so not studied.
I have not noticed fasting having any impact on their size or their prevalence. However, I have never done they kind of extended fasting the other poster has done who has observed an effect. Since I am now interested in extending my fasts I am curious so see if they have any effect on my lipomas, a possibility I had not considered before reading the OP.