I have a habit of zoning out if a topic is not interesting (especially if the topic has been exhausted), and then suddenly unintentionally changing the topic. It could be I have a lack of attention span sometimes. I have been asked if I have ADD/ADHD.
Other times I prefer to just listen and won't contribute to the conversation if there is nothing to add. I have been asked why am I being quiet/is everything all right, etc., which is a surprise because I felt totally fine. One thing I would look for in someone (friends included) is if we are comfortable in silence together, because I wouldn't want to assure them all the time that I am fine, etc.