No reason, and I have no idea why they're still anonymous. I unticked the "this collection is anonymous" tickbox, and can't see anything else that could be causing it.
I contacted AO3 Support and they replied with the following:
"It looks like your work is added to two collections. The collection "Space_Swap" is still anonymous.
To take your work out of the collection:
1) Log in. 2) Edit your work. 3) Scroll down to the Associations section, find the collection name next to the Post to Collections / Challenges option, and click the red ‘X’ next to the name. 4) Post without preview."
It worked perfectly, in case anyone else is having the same problems.
Comments
"It looks like your work is added to two collections. The collection "Space_Swap" is still anonymous.
To take your work out of the collection:
1) Log in.
2) Edit your work.
3) Scroll down to the Associations section, find the collection name next to the Post to Collections / Challenges option, and click the red ‘X’ next to the name.
4) Post without preview."
It worked perfectly, in case anyone else is having the same problems.