Because of course to have fans that love so much your game, to the point they spend time and resources just to create contents to homage your game, is a BAD thing and they should be punished.
Sad thing to read From the above it looks like you are in the clear (assuming you aren't getting payed to make it) Grrr, Imrahil
No. I'm not getting payed for it. I have just read GW's official IP guidelines. It seems like they really don't want people to create fan animations. Fan-fiction, artwork and Fan-sites are fine, but as soon as those pictures start moving, Bang! They accuse you of copyright infringement. Welp! Guess I have to release my animation frame by frame from now on.
I agree that this is unfortunate... and possibly a bad idea. I sort of understand it from a corporate position, but it does cut down on fan involvement and fan advertisement. In my opinion, GW wouldn't have one of their better mini-movies if not for fan animations (i.e. Astartes). I find it similar to how Lucas allowed many fan films of Star Wars, and that furthered a lot of love for the movies (Disney may have squashed that, I don't know). Likewise, Wizards of the Coast had an "open gaming license" for the 3rd edition of D&D (and 5th edition I think) and that was actually one of the better decisions they made. This kind of fan involvement virtually always redirects you back to the home brand.
This. I can't emphasize this enough. I don't know who @ GW decided limiting fan-made media (and outright killing animation) was a good idea. Building off what @Just A Skink said, so many other companies with IP way more popular than GW's have minimal issue with fan-created content (or at least haven't taken any legal action to subdue any of it). I'm sure the argument could be made that said IPs are as big as they are because of all the fan-made media! (And yes, WoTC has an OGL for D&D for 5e.) ...smh @ GW. Just, S.M.D.H.
Don't let it stop you @Paul1748, the worst GW can do is ask you to stop. I doubt they will bother to censor a small animator like you. And I'm guessing, sanity will eventually prevail and GW will relent on this bad policy.