The activation of the Sharingan is not always voluntary, but can also be instinctive or reactionary rather than a user consciously taking the effort to turn on their Sharingan.
Certainly in most cases the Sharingan is intentionally turned on by the user consciously activating the Sharingan, but that isn't always the case.
After all, the initial awakening of the Sharingan is always involuntary and completely reactionary. One can't simply will the Sharingan into existence; the Sharingan awakens once certain conditions are meant and the level of emotional distress is high enough that a Sharingan can activate. But the awakening is a process that a user has no control over, except perhaps for emotional control. Essentially, an Uchiha happens to feel a certain emotion and the Sharingan just happens to accompany those feelings.
Like when Obito finally awakens his Sharingan, it's a reactionary, spur-of-the-moment thing that happens only because his emotional stress is high enough, as Obito found himself in an immediate battle situation where it was either fight or die. Because of the intensity of the stress he was under and the desperate panic to save his friends, Obito awakened his Sharingan. It's a fear/stress response, not an act of Obito consciously figuring out how to awaken the Sharingan.
Moreover, Sarada unknowingly turned on her Sharingan multiple times whenever she was experiencing great distress and sadness, as seen in the Shin arc when her Sharingan activates at several moments of extreme distress, like her feelings of deep sorrow and heartache over her dad's absence from her life, or when she believes that her real mom is Karin, not Sakura, due to the DNA test mix-up.
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I think there are other instances of a user involuntarily activating his Sharingan too, though that isn't necessarily easy to prove, since it's difficult to say for sure which activations are conscious/intentional and which are reactionary.
Personally, I've thought that when Sasuke and Itachi encounter each other in Shippuden at the Akatsuki hideout, Sasuke activating his Sharingan is more reactionary rather than intentional, since Itachi is one of the few people that Sasuke is actually afraid of, and there are few people Sasuke fears more than Itachi (though he tries not to show it). I think when Sasuke finds Itachi, the fear and hate he feels for Itachi is intense enough that it triggers an involuntary reaction of his Sharingan. Though admittedly, this is all personal interpretation, since to me it would have made more sense for Sasuke to have activated his Sharingan before he ever entered the Akatsuki hideout if he was looking for Itachi, but Sasuke fearlessly plunged into the enemy's lair without first activating his Sharingan. Plus the speed and lack of hesitation that causes his Sharingan to flicker on makes me think that it's more an emotional activation than a purposeful one, but again, that's wide open for interpretation - that's just how I see things in that particular scene.
