I think this is fairly trivially explained in most cases by the fact that Wikipedia is an encyclopedia. Almost by construction, nearly every article starts with "X is a Y" (modulo grammatical niceties) where Y is a more general class to which X belongs. In cases when the article doesn't follow that form, the first linked term is usually still explanatory or definitional in some sense and hence more general, and cases where that doesn't apply (e.g. Obstacle) are unusual enough that they don't generally break the cycles. There are no "Wikipedia axioms" that I know of, so this is pretty much guaranteed to end in a cycle looping through a limited set of terms used to talk about abstract concepts. Hence Philosophy.