Have you ever taken a journey in your mind? When you read something or hear a word, a phrase, or you think of someone or an event and it reminds you of something which is related to something else.
You begin to think of people or events related to whatever triggered the thought and before you realize it you are off on a different subject. I admit I do it all the time.
For example, yesterday, while making a mental list of things I had to accomplish for the day, I saw a police car. The car reminded me of an obituary I saw last week. The obituary was of state policeman I once knew in West Virginia. He was injured in a training exercise and had to retire early. So, I wondered why he died so young? Now, what did that have to do with my list of things to accomplish?
As I said, I do it all the time.