The article is pretty short and it mainly says that it is due gas content. The amount of gas seems to be dependent on the presence of certain gut bacteria.

According to the study, naturally about half of the population of mice produce floaters. When you collect stool samples of that half and inject it on the other half, they also start producing floaters. On the other hand, when you sterelize the gut of the floater group, they start producing feces that sink.

Fat content seems not to be related.