Hi there, new here, and I have yet a bit of answer for my question, that's really good!
My configuration: MacBook Air, MacOs BigSur last version, Affinity up to date and Magic mouse 2.
So I had this issue too, I'll will see in the preferences to disable it. And see what happens.
Also, I noticed, and report it :
When I had this problem, it wasn't possible to undo it. I mean if you hit cmd-z, it undo other things I did, but not the rotation in itself.
It's annoying because, if you saved your document just after that (sometimes it happens really quickly), your document is saved with that rotation (and you can't undo it),
There is no way to turn back, or maybe adjust it by rotating again by hand, but it takes time to do so.
The only way to be back at the right version (not rotated) is to close the document, don't save it, and then re-open it.... But if you didn't saved much as this before the rotation, you loose all you did.