After 30 years with a diagnosis of Major Depressive Disorder, my recent specialists realized (at the same time I was diagnosed) that I WASN'T DEPRESSED, but I did suffer from EXTREME anxiety, which causes (me) to have the issues you described.
I am generally calm, cool and collected, and able to handle glitches and emergencies (as long as they don't affect me), but I have also been known to appear (at times of hyper-stress) "aloof," or "unaffected."
What actually happens is that my brain hits and "overload" status (I'm assuming) and I suddenly "flash back" to the instant where I was last supposed to act rationally. So, if I was cooking dinner and a car ran through the house and into the living room, at some point I can only assume I'd be found standing at the stove, trying to finish dinner.
It's not a voluntary action; it's just the point at which everything was going well. I've had situations like that which have caused people to assume I wasn't worried or didn't care, when actually I'm just trying to "process" what's happening, because each decision would require further decisions, and it's difficult to think of all the possible choices and decisions, as well as whether or not they are my responsibility to act at all.
On a more daily basis, if I'm anxious, I tend to withdraw and isolate (and obsess). Often, others have to step in and "shake me out of it," to get me to reattach with my environment. Other times, I just have to think things through (in my head) ...