I find it to be automatic for a lot of people. I know several people that go "mh?" (in english) or "hein?" (in french) when they perfectly understood what you said. It's just a reflex, maybe to give themselves a bit more time to think. It's a bit annoying, I often just wait a second and they start answering

As a teenager I did that too, I always replied (the equivalent in my langague of) "what?" to whatever was said. Just a reflex. Then my aunt pointed it out to me, and I was able to stop doing that. Good thing.

Ah that reminds me of when I was in school, and there was a girl who started every answer with "Okay!...".

The teacher would ask, "What's the capital of X?", and she'd respond confidently (and correctly), "Okay! The capital of X is Y."

Not sure if it was conscious on her part or not. I'm sure it came from the spotlight, since she was otherwise "normal" (not that there necessarily is such a thing).