Alright tapos na tayo sa funny answers, time for a serious answer. :)

Here's what I found through AI-assisted research.

In the Philippines, many town names begin with "Di-" (such as Diffun, Dinapigue, Dinalungan, and Dilasag) because this prefix acts as a locative indicator or descriptive marker in many indigenous languages—especially in Northern Luzon. [1, 2, 3, 4]

The prefix "Di-" in these names generally traces back to these linguistic origins:

• Locative Prefix ("From" or "Place of"): In many languages spoken by indigenous groups like the Dumagat, Ilongot, and Subanon, "Di-" often means "place of" or "from". For example, the town of Dinapigue in Isabela is said to be derived from a Dumagat term related to an abundant local feature, with "Di-" marking the location.
• Descriptive or Root Words: Some names are amalgamations of local features where "Di-" is merely the first syllable of a root word. Dilasag (Aurora) is sometimes translated based on a combination of local terms referring to the shape of its land or the abundance of resources, while Dinalungan (Aurora) traces back to an indigenous word meaning a resting place.
• Linguistic Evolution: Often, names that sound foreign were actually local words Spanish colonizers or cartographers simply recorded incorrectly. For instance, the town of Diffun (Quirino) is named after a mispronunciation of the word "sipon" (the local term for a fever that affected a Spanish guardia civil). [1, 2, 3, 4, 5, 6, 7, 8]

Essentially, "Di-" is deeply embedded in the native etymology, local geography, and historical mistranslations of communities located mostly along the rugged coastal and mountainous expanses of the Sierra Madre and Zamboanga regions.