What is going on in my brain when I switch onto "auto-pilot"?

This mostly happens when I am driving, but it happens at other times, as well. For instance, I was driving to work this morning. I remember turning onto a a certain street and then all of a sudden I was 2 blocks from work. I don’t remember anything that occurred between point A and point B. I must have stopped at a few lights, I even turned a very busy street corner at one point.

How and why does this happen? It’s kinda scary. I feel like I could have been in some major accident, but no. It’s like my body knew what to do without my conscious mind driving it.