It is a derivative work when you transcribe subtitles because you put in your own creative (and copyrightable!) elements into it. You choose how to represent sound effects, how to disentangle crosstalk, how to transcribe dialect, and so on.

So, if the movie creator decided to rip your subtitles off and sell them, you could actually sue the movie creator!

As a practical matter, it makes little difference. Most judges won't parse this finely, and you are equally liable whether under the copying or derivation right.