It's quite common and is often refered to as using your 'lizard brain' I think.
something to do with simple repetitive actions that you are used to doing thus no need to use the conscious brain! so it wanders off for a bit.
It is nothing to worry about particularly as if anything had happened, you would have reverted without even noticing.
If you post the same question in SS, Spin will tell you all about it.
"A man may fight for many things. His country, his principles, his friends, the glistening tear on the cheek of a golden child. Personally, I'd mud-wrestle my own mother for a ton of cash, an amusing clock and a sack of French porn."