Apparently, according to fandom wank, when it's based on the author's own head canon, and involves a variety of things (including aliens, blood-drinking and cannibalism) that were not included in the original canon.
Because What Could be Beter than Howl's Moving Castle with Aliens and Blood Drinking?
I have only a vague memory of 'Howl's Moving Castle from when my great-niece Matty was going through her Japanimation fandom phase. But I trust that the people at Fandom Wank and Yultide know of what the speak.
you really have to read it on the site to get the full flavor.
Also, apparently this is the second year in a row this writer has done this without asking the person he's writing for if they want a cross-over and/or his own personal head canon included in their Yuletide story.
Someone, at one point, throws out the time-worn 'Entitlement!' accusation, but given the way the thing seems to be set up, I think you're allowed to basically get the sort of story you want. Sort of like asking for a nice Doctor Who story and instead getting a rewrite of 'Outlaws of Gor.'