You are not alone in your thought process. I go through that too, especially when things seem idle. I think our brain is used to being preoccupied most of the time, that it tries to find things to do when we are not doing much.
I has also happened to me in the past right before having a panic attack, but those are more racing thoughts that don't make sense. At least when I have the thoughts during idle times, I am at peace and don't feel anxious.
"Don't let your yesterday, ruin your today"