Otoño doesn’t sound much like its English translation, fall (the season). But if we think of the less common synonym, Autumn, then the pattern becomes a bit clearer.

Both come from the Latin for the same, Autumnus. But Latin words with an m‑n sound usually became an ñ sound in Spanish. Think of damn and daño, for example. So the a‑t-m‑n of autumn maps to the o‑t-ñ of otoño!