ODO defines subtitles as both
A subordinate title of a published work or article giving additional information about its content.
Captions displayed at the bottom of a cinema or television screen that translate or transcribe the dialogue or narrative.
The former is (I think) self-evident (sub in Latin means, among other things, under). For example, it is evident that a subtitle follows a title (e.g. in a book). However, I see no connection with the second definition. This is, why are captions called subtitles? Which is the origin of this association? According to the definition above, captions transcribe the main text of a script. Yet, you would not call the main text of a book its subtitles would you?