I apologise for any mistake, but this idiom sounds too modern to be explained by etymology.
I want to dredge below the definition, the first one of which is too violent and doesn't explain the neutral meaning of the second here:
2.Have as the main topic or point of interest:
Source: pp 205, Thinking like a Lawyer, Frederick Schauer
But given that questions of law almost always turn on determinations of fact, and given that determinations of fact are in numerous ways structured by legal rules and by characteristic ways of reasoning, to exclude questions of fact from the topic of legal reasoning seems peculiar.
Answer
In the OED, the definition to which you refer is written thus:
3. turn on or upon (fig.):
a. To hinge upon, depend on, have as the centre or pivot of movement or action.
b. To have as its subject, be about or concerned with, relate to: usually said of conversation or debate.
Definition of “turn” via oed.com (login required)
3.a. appears to be the prior sense of the word, and the earliest recorded usage is in 1661:
They that turn upon this hinge, I mean that receive Procurations upon the ground of Custome.
Source: J. Stephens Hist. Disc. Procur. 26, via oed.com (login required)
The note for 3.b. (the definition you’re interested in) says “App[arently] orig[inally] a development of prec[eding] sense, but often associated with other senses: cf. 28.”
The cross-referenced definition is:
28. a. intr. To direct one's mind, desire, or will to or from some person, thing, or action.
Based on all of that, I conclude that this use of “turn on” to mean “have as the main topic” is based on the metaphor of a hinge. This relatively small, important apparatus is at the center of a larger object’s arc of motion. The presence of other senses of “turn”, like that of changing focus to something, probably influenced this transition and left us in the present state on not even needing to refer to a hinge directly.
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