From what I remember from the pre-UB-05 talk. Slightly different from what Bulbapedia is saying though, but seems to fit this image more.
10 years ago, Anabel appeared from an Ultra Wormhole, with amnesia. (Theories are that she came from the Emerald universe, but that's not the point right now)
Around the same time, a Guzzlord also appeared. As UBs are, it was attracted to Anabel which was a Faller.
Looker, Nanu, and an unnamed third person from Interpol set out to neutralize it, using Anabel as bait.
Something went wrong during the mission, causing the death of the unnamed third person. This post represents that.