CW: drawing of a human skeleton
(since we have emojis of skeletons I guess I don't need to provide a CW but strictly speaking that is a representation of a human corpse which should have a CW so I'm just being cautious)
So yeah, it started with this picture, or specifically Luke Correia's voice-over of it posted in early 2021. As I remember it, I inexplicably misinterpreted "had die" as a clipping of "had diarrhea" — probably because the sentence was implying death by foodborne illness, was preceded by some scatological terms, and I had at that point only recently learned "drop trou" as a clipping of "drop one's trousers", so I was already "primed" to look for similar clippings.
I soon realized that I had misinterpreted the meaning, and "had die" was simply intended as a funny way of saying "died" — but I was still amused by the idea of clipping "have diarrhea" to just "have die", and so I found myself saying "boyyoass how I had die over some [insert food I ate earlier]" to myself in a Skeletor-type voice any time I had that type of night. From there "die" (later respelled as "di", and often in the form "mad di") quickly became my preferred term for loose or liquid stool in most contexts, since it was more humorous and indirect than most other terms for a phenomenon I think we can all agree is a bit gross to talk about.